config: Fix sysfs paths for partitions

config: Fix sysfs paths for partitions

generic/019 was failing with:

./tests/generic/019: line 65: /sys/block/pmem0p2/make-it-fail: No such file or directory

When using a partition, the file needed is located at
/sys/block/pmem0/pmem0p2/make-it-fail.

Rather than attempt to deduce whether a block device is a partition
or not, use the symlinks located in /sys/dev/block/ to find the right
location for the make-it-fail file.

Also change btrfs/088 to use the new _sysfs_dev function.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>