io-mapping: move some code within the include guarded section

It is spurious to have some code out-side the include guard in a .h file. 
Fix it.

Link: https://lkml.kernel.org/r/4dbaf427d4300edba6c6bbfaf4d57493b9bec6ee.1669565241.git.christophe.jaillet@wanadoo.fr


Fixes: 1fbaf8fc ("mm: add a io_mapping_map_user helper")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>