generic: add test for zero range over a file range with many small extents

generic: add test for zero range over a file range with many small extents

Test a fallocate() zero range operation against a large file range for which
there are many small extents allocated. Verify the operation does not fail
and the respective range return zeroes on subsequent reads.

This test is motivated by a bug found on btrfs. The patch that fixes the
bug on btrfs has the following subject:

 "btrfs: fix metadata reservation for fallocate that leads to transaction aborts"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>