snapview-server: use telldir api to get the offset of the dentry

* dirent structure (struct dirent) in NetBSD does not contain the offset
  member. Direct access of offset from struct dirent (used here to copy
  its contents to gf_dirent_t structure after making readdir call) causes
  glusterfs build failures on NetBSD. So like posix does, use telldir api
  to get the offset of the dentry (in this case glfs_telldir).

Change-Id: I72472c16cbf55dd99ea80ba982b0a4205e6ebffb
BUG: 1103591
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/7946
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-by: Emmanuel Dreyfus <manu@netbsd.org>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>