libglusterfs/inode: introduce new APIs for ctx handling

* inode_ctx_reset{0,1,2}() for reseting value1, value2, and both respectively
* inode_ctx_get0() - to get the first value only
* inode_ctx_set0() - to set the first value only
* inode_ctx_get1() - to get the second value only
* inode_ctx_set1() - to set the second value only

Change-Id: I4dfbdac81d6a3f4e5784e060c76edabb1692ce03
Signed-off-by: Amar Tumballi <amarts@redhat.com>
Reviewed-on: http://review.gluster.org/5890
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Anand Avati <avati@redhat.com>