common: fix function _require_batched_discard()

common: fix function _require_batched_discard()

Commit 01d42b7efe3fcb9169befda3072c41d25a87c920 broke the check
for the success status of running fstrim. The [ ] bracets should
have been killed. This made several tests being skipped even when
the test/scratch devices support trim/discard.

For reference:

$ [ fstrim /mnt/ ] || echo foobar
bash: [: fstrim: unary operator expected
foobar

$ fstrim /mnt/ || echo foobar
$ echo $?
0

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