fs/xfs: Ensuring failing to mount sets a grub_errno

fs/xfs: Ensuring failing to mount sets a grub_errno

It was previously possible for grub_xfs_mount() to return NULL without
setting grub_errno if the XFS version was invalid. This resulted in it
being possible for grub_dl_unref() to be called twice allowing the XFS
module to be unloaded while there were still references to it.

Fixing this problem in general by ensuring a grub_errno is set if the
fail label is reached.

Reported-by: B Horn <b@horn.uk>
Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>