Drop a sleepable lock when we plan on sleeping

Drop a sleepable lock when we plan on sleeping

g_io_speedup waits for the completion of the speedup request before proceeding
using biowait(), but check_clear_deps is called with the softdeps lock held
(which is non-sleepable). It's safe to drop this lock around the call to
speedup, so do that.

Submitted by: Peter Holm
Reviewed by: kib@