fs/direct-io.c: minor cleanups in do_blockdev_direct_IO

fs/direct-io.c: minor cleanups in do_blockdev_direct_IO

We already get the block counts and calculate the end block at the
beginning of the function.  Let's use the local variables for
consistency and readability.  No functional changes

[[email protected]: constify the locals to prevent future slipups]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Nikolay Borisov <[email protected]>
Reviewed-by: Jeff Moyer <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Jens Axboe <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>