include/linux/bootmem.h: cleanup the comment for BOOTMEM_ flags

Use BOOTMEM_DEFAULT instead of 0 in the comment.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>