xfstests: exit out if _scratch_mount fails

xfstests: exit out if _scratch_mount fails

I test some of the different mkfs options for btrfs, one set doesn't work
properly with small file systems, so the fs won't mount.  This is fine from a
btrfs point of view, but tests that fail to mount the scratch fs will run
anyway, so if it's a "fill the fs" sort of test this will wreak havoc.  To fix
this just error out of _scratch_mount fails.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>