This is the first in a little batch of patches which address long-held
locks in the VFS/MM layer which are affecting our worst-case scheduling
latency, and are making CONFIG_PREEMPT not very useful.

We end up with a worst-case of 500 microseconds at 500MHz, which is
very good.  Unless you do an exit with lots of mmapped memory.
unmap_page_range() needs work.

Some of these patches also add rescheduling points for non-preemptible
kernels - where I felt that the code path could be long enough to be
perceptible.



Three places in the generic pagecache functions need manual
rescheduling points even for non-preemptible kernels:

- generic_file_read()  (Can hold the CPU for seconds)

- generic_file_write() (ditto)

- filemap_fdatawait().  This won't hold the CPU for so long, but it
  can walk many thousands of pages under the lock.  It needs a lock
  drop and scheduling point for both preemptible and non-preemptible
  kernels.  (This one's a bit ugly...)