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>