cramfs: hide function prototypes behind __KERNEL__ macro

Currently, 3 kernel function prototypes are present in a header
file exported to userland. This patch fixes it.

Signed-off-by: Alexander Shishkin <virtuoso@slind.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>