generic: test fsync after fallocate on a very small file

generic: test fsync after fallocate on a very small file

Test that if we have a very small file, with a size smaller than the
block size, then fallocate a very small range within the block size
but past the file's current size, fsync the file and then power
fail, after mounting the filesystem all the file data is there and
the file size is correct.

This test is motivated by a failure in btrfs where it triggered an
assertion when using the no-holes feature, that is, when running
with MKFS_OPTIONS="-O no-holes". The btrfs issue is fixed by a patch
for the linux kernel titled:

 "Btrfs: fix assertion on fsync of regular file when using no-holes
  feature"

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