clean up kernel_{read,write} & friends v4

[Christoph Hellwig]

Hi Al,

this series fixes a few issues and cleans up the helpers that read from
or write to kernel space buffers, and ensures that we don't change the
address limit if we are using the ->read_iter and ->write_iter methods
that don't need the changed address limit.

I did not add your suggested comments on the instances using
uaccess_kernel as all of them already have comments.  If you have
anything better in mind feel free to throw in additional comments.


Changes since v3:
 - keep call_read_iter/call_write_iter for now
 - don't modify an existing long line
 - update a change log

Changes since v2:
 - picked up a few ACKs

Changes since v1:
 - __kernel_write must not take sb_writers
 - unexport __kernel_write