performance/io-cache: check for non-null gfid before calling inode_path

A new non-linked inode is added to lru list. Hence it might be possible
that gfid might be NULL when inode_dump is called. To pass asserts in
inode_path, we've to check for non-null gfid before invoking that
procedure.

Signed-off-by: Raghavendra G <raghavendra@gluster.com>
Change-Id: Iff14efc6d6e2faa33b9f7a81e0a66f6a947b77ed
BUG: 976189
Reviewed-on: http://review.gluster.org/5241
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>