core: avoid infinite mutual recursion between THIS and gf_log

This caused a stack overflow when (for some unknown reason) pthread_setspecific
was failing.  Therefore __glusterfs_this_location called gf_log which used THIS
which wraps __glusterfs_this_location which . . . you get the idea.  We have to
break the loop somewhere, and we can't reasonably make _gf_log stop using THIS,
so we make __glusterfs_this_location stop using _gf_log.

Change-Id: I79c3ea40dd7980bb8ac76a52cdbf5c057b2e1c3c
Signed-off-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.org/5341
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>