linux-user: Handle short reads in mmap_h_gt_g

linux-user: Handle short reads in mmap_h_gt_g

In particular, if an image has a large bss, we can hit
EOF before reading all host_len bytes of the mapping.

Create a helper, mmap_pread to handle the job for both
the larger block in mmap_h_gt_g itself, as well as the
smaller block in mmap_frag.

Cc: qemu-stable@nongnu.org
Fixes: eb5027ac618 ("linux-user: Split out mmap_h_gt_g")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2504
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240820050848.165253-2-richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>