ocfs2: fix oob in __ocfs2_find_path

syzbot constructed a corrupted image, which resulted in el->l_count from
the b-tree extent block being 0. Since the length of the l_recs array
depends on l_count, reading its member e_blkno triggered the out-of-bounds
access reported by syzbot in [1].

The loop terminates when l_count is 0, similar to when next_free is 0.

[1]
UBSAN: array-index-out-of-bounds in fs/ocfs2/alloc.c:1838:11
index 0 is out of range for type 'struct ocfs2_extent_rec[] __counted_by(l_count)' (aka 'struct ocfs2_extent_rec[]')
Call Trace:
__ocfs2_find_path+0x606/0xa40 fs/ocfs2/alloc.c:1838
ocfs2_find_leaf+0xab/0x1c0 fs/ocfs2/alloc.c:1946
ocfs2_get_clusters_nocache+0x172/0xc60 fs/ocfs2/extent_map.c:418
ocfs2_get_clusters+0x505/0xa70 fs/ocfs2/extent_map.c:631
ocfs2_extent_map_get_blocks+0x202/0x6a0 fs/ocfs2/extent_map.c:678
ocfs2_read_virt_blocks+0x286/0x930 fs/ocfs2/extent_map.c:1001
ocfs2_read_dir_block fs/ocfs2/dir.c:521 [inline]
ocfs2_find_entry_el fs/ocfs2/dir.c:728 [inline]
ocfs2_find_entry+0x3e4/0x2090 fs/ocfs2/dir.c:1120
ocfs2_find_files_on_disk+0xdf/0x310 fs/ocfs2/dir.c:2023
ocfs2_lookup_ino_from_name+0x52/0x100 fs/ocfs2/dir.c:2045
_ocfs2_get_system_file_inode fs/ocfs2/sysfile.c:136 [inline]
ocfs2_get_system_file_inode+0x326/0x770 fs/ocfs2/sysfile.c:112
ocfs2_init_global_system_inodes+0x319/0x660 fs/ocfs2/super.c:461
ocfs2_initialize_super fs/ocfs2/super.c:2196 [inline]
ocfs2_fill_super+0x4432/0x65b0 fs/ocfs2/super.c:993
get_tree_bdev_flags+0x40e/0x4d0 fs/super.c:1691
vfs_get_tree+0x92/0x2a0 fs/super.c:1751
fc_mount fs/namespace.c:1199 [inline]

Link: https://lkml.kernel.org/r/tencent_4D99464FA28D9225BE0DBA923F5DF6DD8C07@qq.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Reported-by: syzbot+151afab124dfbc5f15e6@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=151afab124dfbc5f15e6
Reviewed-by: Heming Zhao <heming.zhao@suse.com>
Acked-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Jun Piao <piaojun@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>