Btrfs: fix regression in btrfs_page_mkwrite() from vm_fault_t conversion

The vm_fault_t conversion commit introduced a ret2 variable for tracking
the integer return values from internal btrfs functions.  It was
sometimes returning VM_FAULT_LOCKED for pages that were actually invalid
and had been removed from the radix.  Something like this:

    ret2 = btrfs_delalloc_reserve_space() // returns zero on success

    lock_page(page)
    if (page->mapping != inode->i_mapping)
	goto out_unlock;

...

out_unlock:
    if (!ret2) {
	    ...
	    return VM_FAULT_LOCKED;
    }

This ends up triggering this WARNING in btrfs_destroy_inode()
    WARN_ON(BTRFS_I(inode)->block_rsv.size);

xfstests generic/095 was able to reliably reproduce the errors.

Since out_unlock: is only used for errors, this fix moves it below the
if (!ret2) check we use to return VM_FAULT_LOCKED for success.

Fixes: a528a241 (btrfs: change return type of btrfs_page_mkwrite to vm_fault_t)
Signed-off-by: Chris Mason <clm@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -9005,13 +9005,14 @@ vm_fault_t btrfs_page_mkwrite(struct vm_fault *vmf)
 
 	unlock_extent_cached(io_tree, page_start, page_end, &cached_state);
 
-out_unlock:
 	if (!ret2) {
 		btrfs_delalloc_release_extents(BTRFS_I(inode), PAGE_SIZE, true);
 		sb_end_pagefault(inode->i_sb);
 		extent_changeset_free(data_reserved);
 		return VM_FAULT_LOCKED;
 	}
+
+out_unlock:
 	unlock_page(page);
 out:
 	btrfs_delalloc_release_extents(BTRFS_I(inode), PAGE_SIZE, (ret != 0));