Convert the callouts back to using mutexes.

I did this wrong - I should've included a state flag for each callout
to see if it was supposed to run or not.  I didn't do that.
Instead, just use mutexes anyway.

Suggested by: jhb
Use the standard powerof2() macro from param.h instead of a local one.

Pointed out by:	 jhb@
MFi386: r274398

  Move NFS and TFTP filesystems before the synthetic filesystems (bzip,
  gzip, and split).
When doing busdma sync ops for BUSDMA_COHERENT memory, there is no need
for cache maintenance operations, but ensure that all prior writes have
reached memory when doing a PREWRITE sync.

Submitted by:	Michal Meloun <meloun@miracle.cz>