vfs_nfs4acl_xattr: add POSIX mode check and reset

vfs_nfs4acl_xattr: add POSIX mode check and reset

The vfs_nfs4acl_xattr VFS module is supposed to work the same as
vfs_acl_xattr|tdb with "ignore system acls" set to true. That is,
filesystem permissions should never restrict access and the actual
access checks are done by smbd in userspace.

To better cope with POSIX mode changes via other protocols (eg NFS) or
local filesystem access, add the following tweaks:

o validate ACL blob: if POSIX mode is not 0777/0666 discard the ACL blob
  from the xattr and synthesize a default ACL based on the POSIX mode

o when setting an ACL, check and reset POSIX mode to 0777/0666

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>