The kernel_read & kernel_write functions have always wrapped the
vfs_read & vfs_write functions respectively. However, they could
not be used by vn_rdwr() since the offset wasn't passed as a
pointer. This prevented us from being able to properly update
the file offset.
Linux 4.14 unexported vfs_read & vfs_write but also changed the
signature of kernel_read & kernel_write to provide the needed
functionality. Use these updated functions when available.
Reviewed-by: Pritam Baral <pritam@pritambaral.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#656Closes#667
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