xattr: always us is_posix_acl_xattr() helper

The is_posix_acl_xattr() helper was added in 0c5fd887

 ("acl: move
idmapped mount fixup into vfs_{g,s}etxattr()") to remove the open-coded
checks for POSIX ACLs. We missed to update two locations. Switch them to
use the helper.

Cc: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Reviewed-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>