[BLKTAP] Fix SMP debugging assert failures in blktap

[BLKTAP] Fix SMP debugging assert failures in blktap
blktap is calling non-atomic kernel functions with irqs disabled, which
hits various kernel debug traps if those are enabled.  The problem is
req_increase(), which takes the pending_free_lock.

This function is currently only ever called from module initialiation,
where it is impossible for it to race against anything else.  Its
companion, req_decrease(), is not called at all.

Fix by removing the offending locking from req_increase() and, while
we're at it, remove req_decrease() entirely.

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