riscv: fix misaligned access handling of C.SWSP and C.SDSP

This is a backport of a fix that was done in OpenSBI: ec0559eb315b
("lib: sbi_misaligned_ldst: Fix handling of C.SWSP and C.SDSP").

Unlike C.LWSP/C.LDSP, these encodings can be used with the zero
register, so checking that the rs2 field is non-zero is unnecessary.

Additionally, the previous check was incorrect since it was checking
the immediate field of the instruction instead of the rs2 field.

Fixes: 956d705 ("riscv: Unaligned load/store handling for M_MODE")
Signed-off-by: Clément Léger <cleger@rivosinc.com>
Link: https://lore.kernel.org/r/20231103090223.702340-1-cleger@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: clementleger

arch/riscv/kernel/traps_misaligned.c

@@ -550,16 +550,14 @@ int handle_misaligned_store(struct pt_regs *regs)
 	} else if ((insn & INSN_MASK_C_SD) == INSN_MATCH_C_SD) {
 		len = 8;
 		val.data_ulong = GET_RS2S(insn, regs);
-	} else if ((insn & INSN_MASK_C_SDSP) == INSN_MATCH_C_SDSP &&
-		   ((insn >> SH_RD) & 0x1f)) {
+	} else if ((insn & INSN_MASK_C_SDSP) == INSN_MATCH_C_SDSP) {
 		len = 8;
 		val.data_ulong = GET_RS2C(insn, regs);
 #endif
 	} else if ((insn & INSN_MASK_C_SW) == INSN_MATCH_C_SW) {
 		len = 4;
 		val.data_ulong = GET_RS2S(insn, regs);
-	} else if ((insn & INSN_MASK_C_SWSP) == INSN_MATCH_C_SWSP &&
-		   ((insn >> SH_RD) & 0x1f)) {
+	} else if ((insn & INSN_MASK_C_SWSP) == INSN_MATCH_C_SWSP) {
 		len = 4;
 		val.data_ulong = GET_RS2C(insn, regs);
 	} else if ((insn & INSN_MASK_C_FSD) == INSN_MATCH_C_FSD) {