mm/bootmem.c: cosmetic improvement of code readability

The obvious number of bits in a byte is replaced by BITS_PER_BYTE macro
in bootmap_bytes()

Link: http://lkml.kernel.org/r/1483781600-5136-1-git-send-email-ondar07@gmail.com


Signed-off-by: Adygzhy Ondar <ondar07@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>