LoongArch: Fix access error when read fault on a write-only VMA

As with most architectures, allow handling of read faults in VMAs that
have VM_WRITE but without VM_READ (WRITE implies READ).

Otherwise, reading before writing a write-only memory will error while
reading after writing everything is fine.

BTW, move the VM_EXEC judgement before VM_READ/VM_WRITE to make logic a
little clearer.

Cc: stable@vger.kernel.org
Fixes: 09cfefb7

 ("LoongArch: Add memory management")
Signed-off-by: Jiantao Shan <shanjiantao@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>