Btrfs: handle ERR_PTR from posix_acl_from_xattr()

posix_acl_from_xattr() returns both ERR_PTRs and null, but it's OK to
pass null values to set_cached_acl()

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>