BKL-removal: remove incorrect comment refering to lock_kernel() from jbd/jbd2

None of the callers of this function does actually take the BKL as far as I
can see.  So remove the comment refering to the BKL.

Signed-off-by: Andi Kleen <ak@suse.de>
Cc: <linux-ext4@vger.kernel.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>