generic: test number of blocks used by a file after mwrite into a hole

generic: test number of blocks used by a file after mwrite into a hole

Test that after doing a memory mapped write to an empty file, a call to
stat(2) reports a non-zero number of used blocks.

This is motivated by a bug in btrfs where the number of blocks used does
not change. It currenly fails on btrfs and it is fixed by a patch that
has the following subject:

  "btrfs: fix missing delalloc new bit for new delalloc ranges"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>