This test mmap()s a file, which means reading the
memory can block, triggering file I/O and FUSE requests.
With GOMAXPROCS=1, we don't have a free thread to handle
those requests, deadlocking the test:

	GOMAXPROCS=1 go test -v -count=1 -run TestCacheControl ./fuse/test

Use mlock() to fix that, also add a warning to the
"fuse" package docs.

Fixes https://github.com/hanwen/go-fuse/issues/261

Change-Id: Ia32fb1826c2be602cf4f0f6cf73956af797c940c