mm/nommu: Fix return type of filemap_map_pages()

mm/nommu: Fix return type of filemap_map_pages()

If CONFIG_MMU is not set (e.g. m68k/m5272c3_defconfig):

    mm/nommu.c:1671:6: error: conflicting types for ‘filemap_map_pages’
     1671 | void filemap_map_pages(struct vm_fault *vmf,
	  |      ^~~~~~~~~~~~~~~~~
    In file included from mm/nommu.c:20:
    ./include/linux/mm.h:2578:19: note: previous declaration of ‘filemap_map_pages’ was here
     2578 | extern vm_fault_t filemap_map_pages(struct vm_fault *vmf,
	  |                   ^~~~~~~~~~~~~~~~~

The signature of filemap_map_pages() was changed, but the nommu
implementation wasn't updated.

Reported-by: noreply@ellerman.id.au
Fixes: f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault() codepaths")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20210128100626.2257638-1-geert@linux-m68k.org
Signed-off-by: Will Deacon <will@kernel.org>