crypto: asymmetric_keys/rsa - Use non-conflicting variable name

arm64:allmodconfig fails to build as follows.

In file included from include/acpi/platform/aclinux.h:74:0,
                 from include/acpi/platform/acenv.h:173,
                 from include/acpi/acpi.h:56,
                 from include/linux/acpi.h:37,
                 from ./arch/arm64/include/asm/dma-mapping.h:21,
                 from include/linux/dma-mapping.h:86,
                 from include/linux/skbuff.h:34,
                 from include/crypto/algapi.h:18,
                 from crypto/asymmetric_keys/rsa.c:16:
include/linux/ctype.h:15:12: error: expected ‘;’, ‘,’ or ‘)’
		before numeric constant
 #define _X 0x40 /* hex digit */
            ^
crypto/asymmetric_keys/rsa.c:123:47: note: in expansion of macro ‘_X’
 static int RSA_I2OSP(MPI x, size_t xLen, u8 **_X)
                                               ^
crypto/asymmetric_keys/rsa.c: In function ‘RSA_verify_signature’:
crypto/asymmetric_keys/rsa.c:256:2: error:
		implicit declaration of function ‘RSA_I2OSP’

The problem is caused by an unrelated include file change, resulting in
the inclusion of ctype.h on arm64. This in turn causes the local variable
_X to conflict with macro _X used in ctype.h.

Fixes: b6197b93 ("arm64 : Introduce support for ACPI _CCA object")
Cc: Suthikulpanit, Suravee <Suravee.Suthikulpanit@amd.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/asymmetric_keys/rsa.c

@@ -120,7 +120,7 @@ static int RSAVP1(const struct public_key *key, MPI s, MPI *_m)
 /*
  * Integer to Octet String conversion [RFC3447 sec 4.1]
  */
-static int RSA_I2OSP(MPI x, size_t xLen, u8 **_X)
+static int RSA_I2OSP(MPI x, size_t xLen, u8 **pX)
 {
 	unsigned X_size, x_size;
 	int X_sign;
@@ -147,7 +147,7 @@ static int RSA_I2OSP(MPI x, size_t xLen, u8 **_X)
 		return -EBADMSG;
 	}
 
-	*_X = X;
+	*pX = X;
 	return 0;
 }