mm/frontswap.c: use non-atomic '__set_bit()' when possible

The 'a' and 'b' bitmaps are local to this function, so no concurrent
access can occur.  So the non-atomic '__set_bit()' can be used to save a
few cycles.

Link: https://lkml.kernel.org/r/e52476da5cee57151745c5c3c934a69798dc6fa4.1638132190.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>