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

module/zfs/zfs_ioctl.c

@@ -159,7 +159,7 @@
 #include <sys/types.h>
 #include <sys/param.h>
 #include <sys/errno.h>
-#include <sys/uio.h>
+#include <sys/uio_impl.h>
 #include <sys/file.h>
 #include <sys/kmem.h>
 #include <sys/cmn_err.h>