block: Change direct_access calling convention

block: Change direct_access calling convention

In order to support accesses to larger chunks of memory, pass in a
'size' parameter (counted in bytes), and return the amount available at
that address.

Add a new helper function, bdev_direct_access(), to handle common
functionality including partition handling, checking the length requested
is positive, checking for the sector being page-aligned, and checking
the length of the request does not pass the end of the partition.

Signed-off-by: Matthew Wilcox <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Reviewed-by: Boaz Harrosh <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>