lib/mpi: mpi_read_buffer(): replace open coded endian conversion

Currently, the endian conversion from CPU order to BE is open coded in
mpi_read_buffer().

Replace this by the centrally provided cpu_to_be*() macros.
Copy from the temporary storage on stack to the destination buffer
by means of memcpy().
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

lib/mpi/mpicoder.c

@@ -21,6 +21,7 @@
 #include <linux/bitops.h>
 #include <linux/count_zeros.h>
 #include <linux/byteorder/generic.h>
+#include <linux/string.h>
 #include "mpi-internal.h"
 
 #define MAX_EXTERN_MPI_BITS 16384
@@ -164,7 +165,13 @@ int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
 		    int *sign)
 {
 	uint8_t *p;
-	mpi_limb_t alimb;
+#if BYTES_PER_MPI_LIMB == 4
+	__be32 alimb;
+#elif BYTES_PER_MPI_LIMB == 8
+	__be64 alimb;
+#else
+#error please implement for this limb size.
+#endif
 	unsigned int n = mpi_get_size(a);
 	int i, lzeros;
 
@@ -187,25 +194,15 @@ int mpi_read_buffer(MPI a, uint8_t *buf, unsigned buf_len, unsigned *nbytes,
 	for (i = a->nlimbs - 1 - lzeros / BYTES_PER_MPI_LIMB,
 			lzeros %= BYTES_PER_MPI_LIMB;
 		i >= 0; i--) {
-		alimb = a->d[i];
 #if BYTES_PER_MPI_LIMB == 4
-		*p++ = alimb >> 24;
-		*p++ = alimb >> 16;
-		*p++ = alimb >> 8;
-		*p++ = alimb;
+		alimb = cpu_to_be32(a->d[i]);
 #elif BYTES_PER_MPI_LIMB == 8
-		*p++ = alimb >> 56;
-		*p++ = alimb >> 48;
-		*p++ = alimb >> 40;
-		*p++ = alimb >> 32;
-		*p++ = alimb >> 24;
-		*p++ = alimb >> 16;
-		*p++ = alimb >> 8;
-		*p++ = alimb;
+		alimb = cpu_to_be64(a->d[i]);
 #else
 #error please implement for this limb size.
 #endif
-
+		memcpy(p, &alimb, BYTES_PER_MPI_LIMB);
+		p += BYTES_PER_MPI_LIMB;
 		if (lzeros > 0) {
 			mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
 			mpi_limb_t *limb2 = (void *)p - sizeof(alimb)