block: disable preemption before using sched_clock()

Commit 9195291e5f05e01d67f9a09c756b8aca8f009089 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>