bvec: make the bvec_iter helpers inline functions

bvec: make the bvec_iter helpers inline functions

The macros are impossible to follow due to the lack of visual type
information and all the braces.  Replace them with inline helpers to
improve on that.  Because the calling conventions are a bit problematic
with a lot of passing structures by value, all the helpers are marked
as __always_inline so that they are force inlined.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <tom.leiming@gmail.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Link: https://patch.msgid.link/20260527151043.2349900-4-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>