mm/filemap.c: provide dummy filemap_page_mkwrite() for NOMMU

Building orangefs on MMU-less machines now results in a link error
because of the newly introduced use of the filemap_page_mkwrite()
function:

  ERROR: "filemap_page_mkwrite" [fs/orangefs/orangefs.ko] undefined!

This adds a dummy version for it, similar to the existing
generic_file_mmap and generic_file_readonly_mmap stubs in the same file,
to avoid the link error without adding #ifdefs in each file system that
uses these.

Link: http://lkml.kernel.org/r/20180409105555.2439976-1-arnd@arndb.de
Fixes: a5135eeab2e5 ("orangefs: implement vm_ops->fault")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Martin Brandenburg <martin@omnibond.com>
Cc: Mike Marshall <hubcap@omnibond.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -2719,7 +2719,6 @@ out:
 	sb_end_pagefault(inode->i_sb);
 	return ret;
 }
-EXPORT_SYMBOL(filemap_page_mkwrite);
 
 const struct vm_operations_struct generic_file_vm_ops = {
 	.fault		= filemap_fault,
@@ -2750,6 +2749,10 @@ int generic_file_readonly_mmap(struct file *file, struct vm_area_struct *vma)
 	return generic_file_mmap(file, vma);
 }
 #else
+int filemap_page_mkwrite(struct vm_fault *vmf)
+{
+	return -ENOSYS;
+}
 int generic_file_mmap(struct file * file, struct vm_area_struct * vma)
 {
 	return -ENOSYS;
@@ -2760,6 +2763,7 @@ int generic_file_readonly_mmap(struct file * file, struct vm_area_struct * vma)
 }
 #endif /* CONFIG_MMU */
 
+EXPORT_SYMBOL(filemap_page_mkwrite);
 EXPORT_SYMBOL(generic_file_mmap);
 EXPORT_SYMBOL(generic_file_readonly_mmap);