linux-user: Use signed lengths in uaccess.c

linux-user: Use signed lengths in uaccess.c

Partially revert 09f679b62dff, but only for the length arguments.
Instead of reverting to long, use ssize_t.  Reinstate the > 0 check
in unlock_user.

Fixes: 09f679b62dff
Reported-by: Coverity (CID 1446711)
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210315204004.2025219-1-richard.henderson@linaro.org>
[lv: remove superfluous semicolon]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>