Use the __ffs/__fls macros to calculate the position and size of the
mask.

Correct type of mask to u32 instead of unsigned long.
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200320020028.1936003-9-nivedita@alum.mit.eduSigned-off-by: Ard Biesheuvel <ardb@kernel.org>