btrfs: change return type of btrfs_page_mkwrite to vm_fault_t

Use the new return type vm_fault_t for fault handler. For now, this is
just documenting that the function returns a VM_FAULT value rather than
an errno. Once all instances are converted, vm_fault_t will become a
distinct type.

Reference commit 1c8f422059ae ("mm: change return type to vm_fault_t")

vmf_error() is the newly introduced inline function in 4.17-rc6.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index f4bf787..118346a 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -3197,7 +3197,7 @@ int btrfs_merge_bio_hook(struct page *page, unsigned long offset,
 			 size_t size, struct bio *bio,
 			 unsigned long bio_flags);
 void btrfs_set_range_writeback(void *private_data, u64 start, u64 end);
-int btrfs_page_mkwrite(struct vm_fault *vmf);
+vm_fault_t btrfs_page_mkwrite(struct vm_fault *vmf);
 int btrfs_readpage(struct file *file, struct page *page);
 void btrfs_evict_inode(struct inode *inode);
 int btrfs_write_inode(struct inode *inode, struct writeback_control *wbc);