linux(4): Return ENOTSUP from xattr syscalls instead of EPERM

FreeBSD does not permits manipulating extended attributes in the system
namespace by unprivileged accounts, even if account has appropriate
privileges to access filesystem object.
In Linux the system namespace is used to preserve posix acls. Some Gnu
coreutils binaries uses posix acls, eg, install, ls. And fails if we
unexpectedly return EPERM error from xattr system calls.

In the other hands, in Linux read and write access to the system
namespace depend on the policy implemented for each filesystem, so we'll
mimics we're a filesystem that prohibits this for unpriveleged accounts.

Approved by: re (gjb)
Reported by: zirias
Tested by: zirias
MFC after: 1 week

(cherry picked from commit 1bfc4574f78653e4b64ac9dd31518c96a17fe52b)
(cherry picked from commit bce9c2e34006dd70fb77a72f8cce1ead8a01db9e)