linux-user: Replace bprm->fd with bprm->src.fd

There are only a couple of uses of bprm->fd remaining.
Migrate to the other field.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/loader.h

@@ -74,7 +74,6 @@ struct linux_binprm {
     char buf[BPRM_BUF_SIZE] __attribute__((aligned));
     ImageSource src;
     abi_ulong p;
-    int fd;
     int e_uid, e_gid;
     int argc, envc;
     char **argv;