fs/buffer: use sleeping lookup in __getblk_slowpath()

Just as with the fast path, call the lookup variant depending
on the gfp flags.

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Link: https://lore.kernel.org/20250515173925.147823-2-dave@stgolabs.net


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