fs: consolidate fsverity_info lookup in buffer.c

fs: consolidate fsverity_info lookup in buffer.c

Look up the fsverity_info once in end_buffer_async_read_io, and then
pass it along to the I/O completion workqueue in
struct postprocess_bh_ctx.

This amortizes the lookup better once it becomes less efficient.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20260202060754.270269-8-hch@lst.de
Signed-off-by: Eric Biggers <ebiggers@kernel.org>