fstests: generic test for fallocate

fstests: generic test for fallocate

Test extent pre-allocation (using fallocate) into a region that already has a
pre-allocated extent that ends beyond the file's size. Verify that if the fs
is unmounted immediately after, the file's size and content are not lost.

This is motivated by a minor issue found in btrfs where the second fallocate
wouldn't update the inode's i_size on disk, fixed by the following btrfs
patch: "Btrfs: add missing inode item update in fallocate()".

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