Minor performance fix for NEON RAID-Z

The NEON code replicates too closely the SSE code, including
a masked 16-bits shift. But NEON, like AltiVec (#9539), has
unsigned 8-bits shift, so use that instead and drop the masking.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Romain Dolbeau <romain.dolbeau@european-processor-initiative.eu>
Closes #9725

module/zfs/vdev_raidz_math_aarch64_neon_common.h

@@ -479,10 +479,8 @@ typedef struct v {
 		/* upper part */					\
 		"and v14.16b," VR0(r) ".16b,v15.16b\n"			\
 		"and v13.16b," VR1(r) ".16b,v15.16b\n"			\
-		"sshr " VR0(r) ".8h," VR0(r) ".8h,#4\n"			\
-		"sshr " VR1(r) ".8h," VR1(r) ".8h,#4\n"			\
-		"and " VR0(r) ".16b," VR0(r) ".16b,v15.16b\n"		\
-		"and " VR1(r) ".16b," VR1(r) ".16b,v15.16b\n"		\
+		"ushr " VR0(r) ".16b," VR0(r) ".16b,#4\n"		\
+		"ushr " VR1(r) ".16b," VR1(r) ".16b,#4\n"		\
 									\
 		"tbl v12.16b,{v10.16b}," VR0(r) ".16b\n"		\
 		"tbl v10.16b,{v10.16b}," VR1(r) ".16b\n"		\