fix up ext2_fs.h for userspace after reservations backport

In commit a686cd89

:

 "Val's cross-port of the ext3 reservations code into ext2."

include/linux/ext2_fs.h got a new function whose return value is only
defined if __KERNEL__ is defined. Putting #ifdef __KERNEL__ around the
function seems to help, patch below.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>