btrfs: test for qgroup reservation leaks with prealloc

btrfs: test for qgroup reservation leaks with prealloc

This test case writes into pre-allocated space, then tries to
fallocate some more within the defined quota limit. Currently
(4.14-rc7) this fails with EDQUOT due to quota reservation leakage
when writing into pre- allocated space.

A possible fix has been sent to the ML as "btrfs: Fix quota
reservation leak on preallocated files"

Signed-off-by: Justin Maggard <jmaggard@netgear.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>