saner calling conventions for nfs_fs_mount_common()

Allow it to take ERR_PTR() for server and return ERR_CAST() of it in
such case.  All callers used to open-code that...

Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>