fuse: fix leaked ENOSYS error on first statx call

FUSE attempts to detect server support for statx by trying it once and
setting no_statx=1 if it fails with ENOSYS, but consider the following
scenario:

- Userspace (e.g. sh) calls stat() on a file
  * succeeds
- Userspace (e.g. lsd) calls statx(BTIME) on the same file
  - request_mask = STATX_BASIC_STATS | STATX_BTIME
  - first pass: sync=true due to differing cache_mask
  - statx fails and returns ENOSYS
  - set no_statx and retry
  - retry sets mask = STATX_BASIC_STATS
  - now mask == cache_mask; sync=false (time_before: still valid)
  - so we take the "else if (stat)" path
  - "err" is still ENOSYS from the failed statx call

Fix this by zeroing "err" before retrying the failed call.

Fixes: d304553 ("fuse: implement statx")
Cc: stable@vger.kernel.org # v6.6
Signed-off-by: Danny Lin <danny@orbstack.dev>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dir.c

@@ -1321,6 +1321,7 @@ static int fuse_update_get_attr(struct inode *inode, struct file *file,
 			err = fuse_do_statx(inode, file, stat);
 			if (err == -ENOSYS) {
 				fc->no_statx = 1;
+				err = 0;
 				goto retry;
 			}
 		} else {