include: linux/fs.h: put declarations in __KERNEL__

include/linux/fs.h contains externs for a bunch of variables.  That obviously
belongs under ifdef __KERNEL__.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>