linux-user: Validate mmap/mprotect prot value

linux-user: Validate mmap/mprotect prot value

The kernel will return -EINVAL for bits set in the prot argument
that are unknown or invalid.  Previously we were simply cropping
out the bits that we care about.

Introduce validate_prot_to_pageflags to perform this check in a
single place between the two syscalls.  Differentiate between
the target and host versions of prot.  Compute the qemu internal
page_flags value at the same time.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200519185645.3915-2-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>