mips: fix compat_sys_lseek syscall

This is almost compatible, but passing a negative offset should result
in a EINVAL error, but on mips o32 compat mode would seek to a large
32-bit byte offset.

Use compat_sys_lseek() to correctly sign-extend the argument.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>