iov_iter: Fix iter_xarray_get_pages{,_alloc}()

The maths at the end of iter_xarray_get_pages() to calculate the actual
size doesn't work under some circumstances, such as when it's been asked to
extract a partial single page.  Various terms of the equation cancel out
and you end up with actual == offset.  The same issue exists in
iter_xarray_get_pages_alloc().

Fix these to just use min() to select the lesser amount from between the
amount of page content transcribed into the buffer, minus the offset, and
the size limit specified.

This doesn't appear to have caused a problem yet upstream because network
filesystems aren't getting the pages from an xarray iterator, but rather
passing it directly to the socket, which just iterates over it.  Cachefiles
*does* do DIO from one to/from ext4/xfs/btrfs/etc. but it always asks for
whole pages to be written or read.

Fixes: 7ff50620 ("iov_iter: Add ITER_XARRAY")
Reported-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Alexander Viro <viro@zeniv.linux.org.uk>
cc: Dominique Martinet <asmadeus@codewreck.org>
cc: Mike Marshall <hubcap@omnibond.com>
cc: Gao Xiang <xiang@kernel.org>
cc: linux-afs@lists.infradead.org
cc: v9fs-developer@lists.sourceforge.net
cc: devel@lists.orangefs.org
cc: linux-erofs@lists.ozlabs.org
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>