x86: mm: consolidate VM_FAULT_RETRY handling

The VM_FAULT_RETRY handling was confusing and incorrect for the case of
returning to kernel mode.  We need to handle the exception table fixup
if we return to kernel mode due to a fatal signal - it will basically
look to the kernel user mode access like the access failed due to the VM
going away from udner it.  Which is correct - the process is dying - and
avoids the whole "repeat endless kernel page faults" case.

Handling the VM_FAULT_RETRY early and in just one place also simplifies
the mmap_sem handling, since once we've taken care of VM_FAULT_RETRY we
know that we can just drop the lock.  The remaining accounting and
possible error handling is thread-local and does not need the mmap_sem.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>