f2fs: fix to account missing .skipped_gc_rwsem

There is a missing place we forgot to account .skipped_gc_rwsem, fix it.

Fixes: 6f8d4455 ("f2fs: avoid fi->i_gc_rwsem[WRITE] lock in f2fs_gc")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>