[BLKTAP] Remove unnecessary TLB flush from blktap driver.

[BLKTAP] Remove unnecessary TLB flush from blktap driver.

blktap_poll is calling tlb_flush_all() in its main ring buffer polling
loop.  This seems to be superfluous: the hypervisor should be
performing
any necessary tlb flushes on grant table operations performed by the
back-end.  Even a simple memory barrier is unnecessary here as the
RING_PUSH_REQUESTS() call performs a wmb() anyway.

And tlb_flush_all() is not exported to modules, so this call prevents
blktap from building as a module.  Just remove it.

Signed-off-by: Stephen Tweedie <sct@redhat.com>