btrfs: simplify error return logic when getting folio at prepare_one_folio()

There's no need to have special logic to return -EAGAIN in case the call
to __filemap_get_folio() fails, because when FGP_NOWAIT is passed to
__filemap_get_folio() it returns ERR_PTR(-EAGAIN) if it needs to do
something that would imply blocking.

The reason we have this logic is from the days before we migrated to the
folio interface, when we called pagecache_get_page() which would return
NULL instead of an error pointer.

So remove this special casing and always return the error that the call
to __filemap_get_folio() returned.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
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

@@ -866,13 +866,9 @@ static noinline int prepare_one_folio(struct inode *inode, struct folio **folio_
 
 again:
 	folio = __filemap_get_folio(inode->i_mapping, index, fgp_flags, mask);
-	if (IS_ERR(folio)) {
-		if (nowait)
-			ret = -EAGAIN;
-		else
-			ret = PTR_ERR(folio);
-		return ret;
-	}
+	if (IS_ERR(folio))
+		return PTR_ERR(folio);
+
 	ret = set_folio_extent_mapped(folio);
 	if (ret < 0) {
 		folio_unlock(folio);