dax: call get_blocks() with create == 1 for write faults to unwritten extents

Currently, __dax_fault() does not call get_blocks() callback with create
argument set, when we got back unwritten extent from the initial
get_blocks() call during a write fault. This is because originally
filesystems were supposed to convert unwritten extents to written ones
using complete_unwritten() callback. Later this was abandoned in favor of
using pre-zeroed blocks however the condition whether get_blocks() needs
to be called with create == 1 remained.

Fix the condition so that filesystems are not forced to zero-out and
convert unwritten extents when get_blocks() is called with create == 0
(which introduces unnecessary overhead for read faults and can be
problematic as the filesystem may possibly be read-only).
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/dax.c

@@ -667,7 +667,7 @@ int __dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
 	if (error)
 		goto unlock_page;
 
-	if (!buffer_mapped(&bh) && !buffer_unwritten(&bh) && !vmf->cow_page) {
+	if (!buffer_mapped(&bh) && !vmf->cow_page) {
 		if (vmf->flags & FAULT_FLAG_WRITE) {
 			error = get_block(inode, block, &bh, 1);
 			count_vm_event(PGMAJFAULT);