f2fs: fix to skip empty sections in f2fs_get_victim

In age-based victim selection (ATGC, AT_SSR, or GC_CB), f2fs_get_victim
can encounter sections with zero valid blocks. This situation often
arises when checkpoint is disabled or due to race conditions between
SIT updates and dirty list management.

In such cases, f2fs_get_section_mtime() returns INVALID_MTIME, which
subsequently triggers a fatal f2fs_bug_on(sbi, mtime == INVALID_MTIME)
in add_victim_entry() or get_cb_cost().

This patch adds a check in f2fs_get_victim's selection loop to skip
sections with no valid blocks. This prevents unnecessary age
calculations for empty sections and avoids the associated kernel panic.
This change also allows removing redundant checks in add_victim_entry().

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>