x86: Fix a #GP from occurring in usermode library's exception handlers

When handling an exception in usermode.c the exception handler #GPs when
executing IRET to return from the exception handler.  This happens
because the stack segment selector does not have the same privilege
level as the return code segment selector.  Set the stack segment
selector to match the code segment selector's privilege level to fix the
issue.

This problem has been disguised in kvm-unit-tests because a #GP
exception handler has been registered with run_in_user() for the tests
that are currently using this feature.  With a #GP exception handler
registered, the first exception will be processed then #GP on the
IRET. The IRET from the second #GP will then succeed, and the subsequent
lngjmp() will restore RSP to a sane value.  But if no #GP handler is
installed, e.g. if a test wants to handle only #ACs, the #GP on the
initial IRET will be fatal.

This is only a problem in 64-bit mode because 64-bit mode
unconditionally pops SS:RSP  (SDM vol 3, 6.14.3 "IRET in IA-32e Mode").
In 32-bit mode SS:RSP is not popped because there is no privilege level
change when executing IRET at the end of the #GP handler.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211214011823.3277011-2-aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>