switch in-kernel read/write calls to kernel_read/write V2

[Christoph Hellwig]

Hi all,

this series moves almost all kernel callers of vfs_read/write and
friends to the kernel_read/write family that includes the set_fs calls
inside the called routines.  The exceptions are the integrity code
(Mimi has a pending series for that one) and the splice readv code
which doesn't fit those helpers.  But at least no modular users
are left.

Changes since V1:
 - update f_pos under lock in ashmem
 - use void * arguments for kernel_read, kernel_write and __kernel_write
 - fix offset usage in cx25821 and target
 - fixed fpos calculation in nommu do_mmap_private