BKL: remove references to lock_kernel from comments

Lock_kernel is gone from the code, so the comments should be updated,
too.  nfsd now uses lock_flocks instead of lock_kernel to protect
against posix file locks.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: J. Bruce Fields <bfields@redhat.com>
Cc: linux-nfs@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>