f2fs: fix to remove unnecessary f2fs_bug_on() to avoid panic

verify_blkaddr() will trigger panic once we inject fault into
f2fs_is_valid_blkaddr(), fix to remove this unnecessary f2fs_bug_on().

Fixes: 18792e64 ("f2fs: support fault injection for f2fs_is_valid_blkaddr()")
Reviewed-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>