Cleaning up uio headers

Making uio_impl.h the common header interface between Linux and FreeBSD
so both OS's can share a common header file. This also helps reduce code
duplication for zfs_uio_t for each OS.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Brian Atkinson <batkinson@lanl.gov>
Closes #11622

include/sys/zfs_context.h

@@ -72,6 +72,7 @@ extern "C" {
 #include <sys/trace.h>
 #include <sys/procfs_list.h>
 #include <sys/mod.h>
+#include <sys/uio_impl.h>
 #include <sys/zfs_context_os.h>
 #else /* _KERNEL || _STANDALONE */