iov_iter: Convert iterate*() to inline funcs

Convert the iov_iter iteration macros to inline functions to make the code
easier to follow.

The functions are marked __always_inline as we don't want to end up with
indirect calls in the code. This, however, leaves dealing with ->copy_mc
in an awkard situation since the step function (memcpy_from_iter_mc())
needs to test the flag in the iterator, but isn't passed the iterator.
This will be dealt with in a follow-up patch.

The variable names in the per-type iterator functions have been harmonised
as much as possible and made clearer as to the variable purpose.

The iterator functions are also moved to a header file so that other
operations that need to scan over an iterator can be added. For instance,
the rbd driver could use this to scan a buffer to see if it is all zeros
and libceph could use this to generate a crc.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/3710261.1691764329@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/855.1692047347@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/20230816120741.534415-1-dhowells@redhat.com/ # v3
Link: https://lore.kernel.org/r/20230925120309.1731676-8-dhowells@redhat.com
cc: Alexander Viro <viro@zeniv.linux.org.uk>
cc: Jens Axboe <axboe@kernel.dk>
cc: Christoph Hellwig <hch@lst.de>
cc: Christian Brauner <christian@brauner.io>
cc: Matthew Wilcox <willy@infradead.org>
cc: Linus Torvalds <torvalds@linux-foundation.org>
cc: David Laight <David.Laight@ACULAB.COM>
cc: linux-block@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Signed-off-by: Christian Brauner <brauner@kernel.org>