fs/buffer: avoid redundant lookup in getblk slowpath

__getblk_slow() already implies failing a first lookup
as the fastpath, so try to create the buffers immediately
and avoid the redundant lookup. This saves 5-10% of the
total cost/latency of the slowpath.

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Link: https://lore.kernel.org/20250515173925.147823-3-dave@stgolabs.net


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>