MIPS: strnlen_user.S: Fix a CPU_DADDI_WORKAROUNDS regression

Correct a regression introduced with 8453eebd [MIPS: Fix strnlen_user()
return value in case of overlong strings.] causing assembler warnings
and broken code generated in __strnlen_kernel_nocheck_asm:

arch/mips/lib/strnlen_user.S: Assembler messages:
arch/mips/lib/strnlen_user.S:64: Warning: Macro instruction expanded into multiple instructions in a branch delay slot

with the CPU_DADDI_WORKAROUNDS option set, resulting in the function
looping indefinitely upon mounting NFS root.

Use conditional assembly to avoid a microMIPS code size regression.
Using $at unconditionally would cause such a regression as there are no
16-bit instruction encodings available for ALU operations using this
register.  Using $v1 unconditionally would produce short microMIPS
encodings, but would prevent this register from being used across calls
to this function.

The extra LI operation introduced is free, replacing a NOP originally
scheduled into the delay slot of the branch that follows.
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10205/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/lib/strnlen_user.S

@@ -34,7 +34,12 @@ LEAF(__strnlen_\func\()_asm)
 FEXPORT(__strnlen_\func\()_nocheck_asm)
 	move		v0, a0
 	PTR_ADDU	a1, a0			# stop pointer
-1:	beq		v0, a1, 1f		# limit reached?
+1:
+#ifdef CONFIG_CPU_DADDI_WORKAROUNDS
+	.set		noat
+	li		AT, 1
+#endif
+	beq		v0, a1, 1f		# limit reached?
 .ifeqs "\func", "kernel"
 	EX(lb, t0, (v0), .Lfault\@)
 .else
@@ -42,7 +47,13 @@ FEXPORT(__strnlen_\func\()_nocheck_asm)
 .endif
 	.set		noreorder
 	bnez		t0, 1b
-1:	 PTR_ADDIU	v0, 1
+1:
+#ifndef CONFIG_CPU_DADDI_WORKAROUNDS
+	 PTR_ADDIU	v0, 1
+#else
+	 PTR_ADDU	v0, AT
+	.set		at
+#endif
 	.set		reorder
 	PTR_SUBU	v0, a0
 	jr		ra