protocol/client: change log level for lookup

Problem:
On 3.5 branch, http://review.gluster.org/8294 causes the server to return ENOENT
(as opposed to ESTALE in master branch) if file does not exist. When AFR does
entry self-heals (either from the mount or shd or by the `heal info` command), it
does a gfid-lookup with loc.name == NULL, causing the corresponding log file to be
flooded with messages like this:

[2014-10-15 11:12:57.405428] W [client-rpc-fops.c:2761:client3_3_lookup_cbk]
0-testvol-client-1: remote operation failed: No such file or directory. Path:
<gfid:760b4427-2fb9-4a67-9f55-e8e8d78e452f>
(760b4427-2fb9-4a67-9f55-e8e8d78e452f)

Fix:
Change log level for ENOENT and ESTALE errors to DEBUG

Change-Id: Ideb88d9cb609d077e02efe703cd28155985d7513
BUG: 1153904
Signed-off-by: Ravishankar N <root@ravi3.(none)>
Reviewed-on: http://review.gluster.org/8937
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>