block: disable preemption before using sched_clock()

Commit 9195291e added calls to
sched_clock() from preemptible code. sched_clock() is both the
wrong interface AND cannot be called without preempt disabled.

Apply a temporary fix to get rid of the warnings, a real patch
is in the works.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>