mips-linux-user: Adjust names in mips_syscall_args

The name field of MIPS_SYS isn't actually used; it's just documentation.
But adjust the umount entries to match mips/syscall_nr.h anyway.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>