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/spa_config.c

@@ -32,7 +32,6 @@
 #include <sys/fm/fs/zfs.h>
 #include <sys/spa_impl.h>
 #include <sys/nvpair.h>
-#include <sys/uio.h>
 #include <sys/fs/zfs.h>
 #include <sys/vdev_impl.h>
 #include <sys/zfs_ioctl.h>
@@ -40,6 +39,7 @@
 #include <sys/sunddi.h>
 #include <sys/zfeature.h>
 #include <sys/zfs_file.h>
+#include <sys/zfs_context.h>
 #ifdef _KERNEL
 #include <sys/zone.h>
 #endif