Add missing refpurpose of vfs_linux_xfs_sgid manpage

[github at samba.org]

New comment by sathieu on Samba Github repository

https://github.com/samba-team/samba/pull/172#issuecomment-386085156
Comment:
According to https://web.archive.org/web/20160320180937/http://oss.sgi.com/bugzilla/show_bug.cgi?id=280:
> this should be fixed in kernel 3.11 by commit 42c49d7f249c2487f36d3314753d5d8ebcee8249

And according to https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=42c49d7f249c2487f36d3314753d5d8ebcee8249, the bug was during mkdir when a default acl was set.

Proposed refpurpose:
```
Workaround XFS sgid bit not inherited during mkdir with default acls on Linux older than 3.11
```
I'm not english native. Any rewording welcome!