clean up kernel_{read,write} & friends v5

[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 v4:
 - warn on calling __kernel_write on files not open for write
 - add a FMODE_READ check and warning in __kernel_read
 - add a new patch to remove kernel_readv
 - stop preferring the iter variants if normal read/write is
   present

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

Diffstat:
 fs/autofs/waitq.c            |    2 
 fs/cachefiles/rdwr.c         |    2 
 fs/read_write.c              |  171 ++++++++++++++++++++++++++-----------------
 fs/splice.c                  |   53 +++----------
 include/linux/fs.h           |    4 -
 net/bpfilter/bpfilter_kern.c |    2 
 security/integrity/iint.c    |   14 ---
 7 files changed, 125 insertions(+), 123 deletions(-)