xfstests: Add tmpfs support

xfstests: Add tmpfs support

This is just a simple patch to get the tmpfs working as a target file
system. The patch copies the way nfs is handled in xfstests.

I didn't change the xfstests logic to recognize a proper SCRATCH_DEV.
Hence, the SCRATCH_DEV for tmpfs should be in nfs form (with ':' sign
in it) in order for this to work properly.

Signed-off-by: Boris Ranto <branto@redhat.com>
Signed-off-by: Junho Ryu <jayr@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>