MIPS: Drop spurious __unused in struct compat_flock

MIPS' struct compat_flock doesn't match the 32-bit struct flock, as it
has an extra short __unused before pad[4], which combined with alignment
increases the size to 40 bytes compared with struct flock's 36 bytes.

Since commit 8c6657c ("Switch flock copyin/copyout primitives to
copy_{from,to}_user()"), put_compat_flock() writes the full compat_flock
struct to userland, which results in corruption of the userland word
after the struct flock when running 32-bit userlands on 64-bit kernels.

This was observed to cause a bus error exception when starting Firefox
on Debian 8 (Jessie).

Reported-by: Peter Mamonov <[email protected]>
Signed-off-by: James Hogan <[email protected]>
Tested-by: Peter Mamonov <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Al Viro <[email protected]>
Cc: [email protected]
Cc: <[email protected]> # 4.13+
Patchwork: https://patchwork.linux-mips.org/patch/18646/

Author: amalon

arch/mips/include/asm/compat.h

@@ -86,7 +86,6 @@ struct compat_flock {
 	compat_off_t	l_len;
 	s32		l_sysid;
 	compat_pid_t	l_pid;
-	short		__unused;
 	s32		pad[4];
 };