btrfs: simplify early error checking in btrfs_page_mkwrite()

We have this entangled error checks early at btrfs_page_mkwrite():

1) Try to reserve delalloc space by calling btrfs_delalloc_reserve_space()
   and storing the return value in the ret2 variable;

2) If the reservation succeed, call file_update_time() and store the
   return value in ret2 and also set the local variable 'reserved' to
   true (1);

3) Then do an error check on ret2 to see if any of the previous calls
   failed and if so, jump either to the 'out' label or to the
   'out_noreserve' label, depending on whether 'reserved' is true or
   not.

This is unnecessarily complex. Instead change this to a simpler and
more straightforward approach:

1) Call btrfs_delalloc_reserve_space(), if that returns an error jump to
   the 'out_noreserve' label;

2) The call file_update_time() and if that returns an error jump to the
   'out' label.

Like this there's less nested if statements, no need to use a local
variable to track if space was reserved and if statements are used only
to check errors.

Also move the call to extent_changeset_free() out of the 'out_noreserve'
label and under the 'out' label  since the changeset is allocated only if
the call to reserve delalloc space succeeded.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/file.c

@@ -1843,7 +1843,6 @@ static vm_fault_t btrfs_page_mkwrite(struct vm_fault *vmf)
 	size_t fsize = folio_size(folio);
 	vm_fault_t ret;
 	int ret2;
-	int reserved = 0;
 	u64 reserved_space;
 	u64 page_start;
 	u64 page_end;
@@ -1866,17 +1865,17 @@ static vm_fault_t btrfs_page_mkwrite(struct vm_fault *vmf)
 	 */
 	ret2 = btrfs_delalloc_reserve_space(BTRFS_I(inode), &data_reserved,
 					    page_start, reserved_space);
-	if (!ret2) {
-		ret2 = file_update_time(vmf->vma->vm_file);
-		reserved = 1;
-	}
 	if (ret2) {
 		ret = vmf_error(ret2);
-		if (reserved)
-			goto out;
 		goto out_noreserve;
 	}
 
+	ret2 = file_update_time(vmf->vma->vm_file);
+	if (ret2) {
+		ret = vmf_error(ret2);
+		goto out;
+	}
+
 	/* Make the VM retry the fault. */
 	ret = VM_FAULT_NOPAGE;
 again:
@@ -1972,9 +1971,9 @@ static vm_fault_t btrfs_page_mkwrite(struct vm_fault *vmf)
 	btrfs_delalloc_release_extents(BTRFS_I(inode), fsize);
 	btrfs_delalloc_release_space(BTRFS_I(inode), data_reserved, page_start,
 				     reserved_space, true);
+	extent_changeset_free(data_reserved);
 out_noreserve:
 	sb_end_pagefault(inode->i_sb);
-	extent_changeset_free(data_reserved);
 	return ret;
 }