zfs: Implement zfs_fallocate

1) zfs_fallocate is ported from ZFS on Linux along with convoff.
zfs_fallocate being limited to FALLOC_FL_PUNCH_HOLE alone, though
not useful as fallocate require FALLOC_FL_PUNCH_HOLE to be ORed
with FALLOC_FL_KEEP_SIZE.

2) (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE) semantics is implemented
in zfs_fallocate. Relevant changes were introduced into zfs_space.

3) From my conversation with ZFS folks, I realized that FALLOC_FL_KEEP_SIZE
alone could be implemented, but it will probably take a long time, and wouldn't
give us the proportional value.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>