nfsd: Export get_task_comm for nfsd

The git://linux-nfs.org/~bfields/linux.git nfsd-next branch doesn't
compile when nfsd is a module with the following error:

   ERROR: "get_task_comm" [fs/nfsd/nfsd.ko] undefined!

Replace the get_task_comm call with direct comm access, which is
safe for current.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>