btrfs: accessors: use type sizeof constants directly

btrfs: accessors: use type sizeof constants directly

Now unit_size is used only once, so use it directly in 'part'
calculation. Don't cache sizeof(type) in a variable. While this is a
compile-time constant, forcing the type 'int' generates worse code as it
leads to additional conversion from 32 to 64 bit type on x86_64.

The sizeof() is used only a few times and it does not make the code that
harder to read, so use it directly and let the compiler utilize the
immediate constants in the context it needs. The .ko code size slightly
increases (+50) but further patches will reduce that again.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>