iov_iter: Introduce nofault flag to disable page faults

Introduce a new nofault flag to indicate to iov_iter_get_pages not to
fault in user pages.

This is implemented by passing the FOLL_NOFAULT flag to get_user_pages,
which causes get_user_pages to fail when it would otherwise fault in a
page. We'll use the ->nofault flag to prevent iomap_dio_rw from faulting
in pages when page faults are not allowed.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

include/linux/uio.h

@@ -35,6 +35,7 @@ struct iov_iter_state {
 
 struct iov_iter {
 	u8 iter_type;
+	bool nofault;
 	bool data_source;
 	size_t iov_offset;
 	size_t count;