xtensa: don't leave invalid TLB entry in fast_store_prohibited

When fast_store_prohibited needs to go to the C-level exception handler
it leaves TLB entry that caused page fault in the TLB. If the faulting
task gets switched to a different CPU and completes page table update
there the TLB entry will get out of sync with the page table which may
cause a livelock on access to that page.
Invalidate faulting TLB entry on a slow path exit from the
fast_store_prohibited.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>