Linux 4.20 compat: wrapper function for iov_iter type access

An iov_iter_type() function to access the "type" member of the struct
iov_iter was added at one point. Move the conditional logic to decide
which method to use for accessing it into a macro and simplify the
zpl_uio_init code.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Coleman Kane <ckane@colemankane.org>
Closes #15100