metag/uaccess: Check access_ok in strncpy_from_user

The metag implementation of strncpy_from_user() doesn't validate the src
pointer, which could allow reading of arbitrary kernel memory. Add a
short access_ok() check to prevent that.

Its still possible for it to read across the user/kernel boundary, but
it will invariably reach a NUL character after only 9 bytes, leaking
only a static kernel address being loaded into D0Re0 at the beginning of
__start, which is acceptable for the immediate fix.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-metag@vger.kernel.org
Cc: stable@vger.kernel.org