posix_acl: Add set_posix_acl

Factor out part of posix_acl_xattr_set into a common function that takes
a posix_acl, which nfsd can also call.

The prototype already exists in include/linux/posix_acl.h.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Cc: stable@vger.kernel.org
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
