btrfs: add function _require_fssum()

btrfs: add function _require_fssum()

To avoid repeating detection of fssum presence in many btrfs tests, as
suggested by Dave Chinner.

Also exported the variable "here" from the main control script, to avoid
repeating its declaration in every single testcase file. Also removed the
declaration of "here" from btrfs test cases that require the fssum program
only. Removing it from all other test cases will be a separate change.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>