Prevent segfaults in SSE optimized Fletcher-4

In some cases, the compiler was not respecting the GNU aligned
attribute for stack variables in 35a76a0. This was resulting in
a segfault on CentOS 6.7 hosts using gcc 4.4.7-17.  This issue
was fixed in gcc 4.6.

To prevent this from occurring, use unaligned loads and stores
for all stack and global memory references in the SSE optimized
Fletcher-4 code.

Disable zimport testing against master where this flaw exists:

TEST_ZIMPORT_VERSIONS="installed"

Signed-off-by: Tyler J. Stachecki <stachecki.tyler@gmail.com>
Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#4862

module/zcommon/zfs_fletcher_sse.c

@@ -69,12 +69,12 @@ fletcher_4_sse2_fini(zio_cksum_t *zcp)
 	struct zfs_fletcher_sse_array a, b, c, d;
 	uint64_t A, B, C, D;
 
-	asm volatile("movdqa %%xmm0, %0":"=m" (a.v));
-	asm volatile("movdqa %%xmm1, %0":"=m" (b.v));
+	asm volatile("movdqu %%xmm0, %0":"=m" (a.v));
+	asm volatile("movdqu %%xmm1, %0":"=m" (b.v));
 	asm volatile("psllq $0x2, %xmm2");
-	asm volatile("movdqa %%xmm2, %0":"=m" (c.v));
+	asm volatile("movdqu %%xmm2, %0":"=m" (c.v));
 	asm volatile("psllq $0x3, %xmm3");
-	asm volatile("movdqa %%xmm3, %0":"=m" (d.v));
+	asm volatile("movdqu %%xmm3, %0":"=m" (d.v));
 
 	kfpu_end();
 
@@ -168,7 +168,7 @@ fletcher_4_ssse3_byteswap(const void *buf, uint64_t size, zio_cksum_t *unused)
 	const uint64_t *ip = buf;
 	const uint64_t *ipend = (uint64_t *)((uint8_t *)ip + size);
 
-	asm volatile("movdqa %0, %%xmm7"::"m" (mask));
+	asm volatile("movdqu %0, %%xmm7"::"m" (mask));
 	asm volatile("pxor %xmm4, %xmm4");
 
 	for (; ip < ipend; ip += 2) {