filestore: use FL_ALLOC_PUNCH_HOLE to zero, when available

filestore: use FL_ALLOC_PUNCH_HOLE to zero, when available

First try the FL_ALLOC_PUNCH_HOLE fallocate() flag.  If we get EOPNOTSUPP,
fall back to writing zeros.

Check for fallocate(2) with configure.  Also, avoid this if we are not
Linux, since I'm not sure about the hard-coded FL_ALLOC_PUNCH_HOLE being
correct on other platforms.

Signed-off-by: Sage Weil <sage@newdream.net>
Reviewed-by: Samuel Just <samuel.just@dreamhost.com>