arm64: insn: Fix ldadd instruction encoding

GCC 8.1.0 reports that the ldadd instruction encoding, recently added to
insn.c, doesn't match the mask and couldn't possibly be identified:

 linux/arch/arm64/include/asm/insn.h: In function 'aarch64_insn_is_ldadd':
 linux/arch/arm64/include/asm/insn.h:280:257: warning: bitwise comparison always evaluates to false [-Wtautological-compare]

Bits [31:30] normally encode the size of the instruction (1 to 8 bytes)
and the current instruction value only encodes the 4- and 8-byte
variants. At the moment only the BPF JIT needs this instruction, and
doesn't require the 1- and 2-byte variants, but to be consistent with
our other ldr and str instruction encodings, clear the size field in the
insn value.

Fixes: 34b8ab09 ("bpf, arm64: use more scalable stadd over ldxr / stxr loop in xadd")
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Reported-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/insn.h

@@ -277,7 +277,7 @@ __AARCH64_INSN_FUNCS(adrp,	0x9F000000, 0x90000000)
 __AARCH64_INSN_FUNCS(prfm,	0x3FC00000, 0x39800000)
 __AARCH64_INSN_FUNCS(prfm_lit,	0xFF000000, 0xD8000000)
 __AARCH64_INSN_FUNCS(str_reg,	0x3FE0EC00, 0x38206800)
-__AARCH64_INSN_FUNCS(ldadd,	0x3F20FC00, 0xB8200000)
+__AARCH64_INSN_FUNCS(ldadd,	0x3F20FC00, 0x38200000)
 __AARCH64_INSN_FUNCS(ldr_reg,	0x3FE0EC00, 0x38606800)
 __AARCH64_INSN_FUNCS(ldr_lit,	0xBF000000, 0x18000000)
 __AARCH64_INSN_FUNCS(ldrsw_lit,	0xFF000000, 0x98000000)