Patch to allow disabling devices for testing failure/recovery scenarios.

Patch to allow disabling devices for testing failure/recovery scenarios.
By itself this patch does not enable failure testing, but when it is
applied to the kernel, the OBD_FAIL_WRITE() call will activate it.

Patch currently supports loopback, IDE, and UML ubd failures (ignore patch
reject for ubc.c if not using UML).

Enable MDS write failure testing via "echo <num> > /proc/sys/lustre/fail_loc"
where <num> is the number of a write failure breakpoint (OBD_FAIL_MDS_*_WRITE
as defined in the enum in obd_support.h).