f2fs: fix age extent cache insertion skip on counter overflow

The age extent cache uses last_blocks (derived from
allocated_data_blocks) to determine data age. However, there's a
conflict between the deletion
marker (last_blocks=0) and legitimate last_blocks=0 cases when
allocated_data_blocks overflows to 0 after reaching ULLONG_MAX.

In this case, valid extents are incorrectly skipped due to the
"if (!tei->last_blocks)" check in __update_extent_tree_range().

This patch fixes the issue by:
1. Reserving ULLONG_MAX as an invalid/deletion marker
2. Limiting allocated_data_blocks to range [0, ULLONG_MAX-1]
3. Using F2FS_EXTENT_AGE_INVALID for deletion scenarios
4. Adjusting overflow age calculation from ULLONG_MAX to (ULLONG_MAX-1)

Reproducer (using a patched kernel with allocated_data_blocks
initialized to ULLONG_MAX - 3 for quick testing):

Step 1: Mount and check initial state
# dd if=/dev/zero of=/tmp/test.img bs=1M count=100
# mkfs.f2fs -f /tmp/test.img
# mkdir -p /mnt/f2fs_test
# mount -t f2fs -o loop,age_extent_cache /tmp/test.img /mnt/f2fs_test
# cat /sys/kernel/debug/f2fs/status | grep -A 4 "Block Age"
Allocated Data Blocks: 18446744073709551612 # ULLONG_MAX - 3
Inner Struct Count: tree: 1(0), node: 0

Step 2: Create files and write data to trigger overflow
# touch /mnt/f2fs_test/{1,2,3,4}.txt; sync
# cat /sys/kernel/debug/f2fs/status | grep -A 4 "Block Age"
Allocated Data Blocks: 18446744073709551613 # ULLONG_MAX - 2
Inner Struct Count: tree: 5(0), node: 1

# dd if=/dev/urandom of=/mnt/f2fs_test/1.txt bs=4K count=1; sync
# cat /sys/kernel/debug/f2fs/status | grep -A 4 "Block Age"
Allocated Data Blocks: 18446744073709551614 # ULLONG_MAX - 1
Inner Struct Count: tree: 5(0), node: 2

# dd if=/dev/urandom of=/mnt/f2fs_test/2.txt bs=4K count=1; sync
# cat /sys/kernel/debug/f2fs/status | grep -A 4 "Block Age"
Allocated Data Blocks: 18446744073709551615 # ULLONG_MAX
Inner Struct Count: tree: 5(0), node: 3

# dd if=/dev/urandom of=/mnt/f2fs_test/3.txt bs=4K count=1; sync
# cat /sys/kernel/debug/f2fs/status | grep -A 4 "Block Age"
Allocated Data Blocks: 0 # Counter overflowed!
Inner Struct Count: tree: 5(0), node: 4

Step 3: Trigger the bug - next write should create node but gets skipped
# dd if=/dev/urandom of=/mnt/f2fs_test/4.txt bs=4K count=1; sync
# cat /sys/kernel/debug/f2fs/status | grep -A 4 "Block Age"
Allocated Data Blocks: 1
Inner Struct Count: tree: 5(0), node: 4

Expected: node: 5 (new extent node for 4.txt)
Actual: node: 4 (extent insertion was incorrectly skipped due to
last_blocks = allocated_data_blocks = 0 in __get_new_block_age)

After this fix, the extent node is correctly inserted and node count
becomes 5 as expected.

Fixes: 71644dff4811 ("f2fs: add block_age-based extent cache")
Cc: stable@kernel.org
Signed-off-by: Xiaole He <hexiaole1994@126.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>