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: NMatthew Wilcox <matthew.r.wilcox@intel.com>
Reviewed-by: NJan Kara <jack@suse.cz>
Reviewed-by: NBoaz Harrosh <boaz@plexistor.com>
Signed-off-by: NJens Axboe <axboe@fb.com>