Rather than using a zero tuple to end the table, use a macro
to apply ARRAY_SIZE and pass that on to the convert functions.

This fixes two bugs in which the conversion functions required
that both the target and host masks be non-zero in order to
continue, rather than require both target and host masks be
zero in order to terminate.

This affected mmap_flags_tbl when the host does not support
all of the flags we wish to convert (e.g. MAP_UNINITIALIZED).
Mapping these flags to zero is good enough, and matches how
the kernel ignores bits that are unknown.

Fixes: 4b840f96 ("linux-user: Populate more bits in mmap_flags_tbl")
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>