mount/fuse: Implement forget in cbks for fuse.

With the introduction of inode_ctx_set in fuse as part of  2991503d014,
forget cbk gets called for fuse xlator. Though nothing needs to be done
inf forget_cbk, excessive log messages of the following kind are observed:

[2013-09-16 06:09:50.758063] W [defaults.c:1331:default_forget]
(-->/usr/local/lib/glusterfs/3git/xlator/mount/fuse.so(+0xa1f2) [0x7f51432781f2]
(-->/usr/local/lib/libglusterfs.so.0(inode_unref+0x3c) [0x7f5144e5
816c] (-->/usr/local/lib/libglusterfs.so.0(+0x2d061) [0x7f5144e58061]))) 0-fuse:
xlator does not implement forget_cbk

This patch prevents such log messages from being seen.
Signed-off-by: Vijay Bellur <vbellur@redhat.com>
BUG: 979910
Change-Id: Ie5874138f46822b10ff4213bd1134d78330ec460
Reviewed-on: http://review.gluster.org/5932
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5975
Tested-by: Gluster Build System <jenkins@build.gluster.com>