The iret instruction may generate #np and #ss fault, besides #gp.
When returning to usermode, the handler for that exceptions is also
executed with wrong gs base.  Handle all three possible faults in the
same way, checking for iret fault, and performing full iret.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275833 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f