btrfs: cleanup, rename a few variables in btrfs_read_sys_array
btrfs: cleanup, rename a few variables in btrfs_read_sys_array

There's a pointer to buffer, integer offset and offset passed as
pointer, try to find matching names for them.

Signed-off-by: David Sterba <a2ce9d316ca04d17b520237d2846a218b8284e52@suse.cz>
Signed-off-by: Chris Mason <e2e8c5702d8ed5f565988a1cf5bb35c9ce199e5e@fb.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs