iomap: introduce a full map advance helper

Various iomap_iter_advance() calls advance by the full mapping
length and thus have no need for the current length input or
post-advance remaining length output from the standard advance
function. Add an iomap_iter_advance_full() helper to clean up these
cases.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/r/20250224144757.237706-13-bfoster@redhat.com


Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>