ramfs: move RAMFS_MAGIC to include/linux/magic.h

initramfs userspace likes to use this magic number.

Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: maximilian attems <max@stro.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>