fs: fix include/percpu-rwsem.h export error

We get the following export error on the include file:

usr/include/linux/fs.h:13: included file 'linux/percpu-rwsem.h' is not exported

Move the include inside the __KERNEL__ section.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jens Axboe <axboe@kernel.dk>