gfapi: fix glfs_readlink()

glfs_readlink() is supposed to memcpy the buffer pointer
returned by syncop_readlink(). Fix it.

Change-Id: I5936b07abbd93cf02b354233dc60f6623e30a38b
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5439
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra Talur <rtalur@redhat.com>