zerofill: Change the type of len argument of glfs_zerofill() to off_t

glfs_zerofill() can be potentially called to zero-out entire file and
hence allow for bigger value of length parameter.

Change-Id: I75f1d11af298915049a3f3a7cb3890a2d72fca63
BUG: 1028673
Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-on: http://review.gluster.org/6266
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: M. Mohan Kumar <mohan@in.ibm.com>
Tested-by: M. Mohan Kumar <mohan@in.ibm.com>
Reviewed-by: Anand Avati <avati@redhat.com>