UPSTREAM: debugfs_mknod(): get rid useless arguments

dev is always zero, dir was only used to get its ->i_sb, which is
equal to ->d_sb of dentry...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Bug: 31856701
Change-Id: I0cd2935b19dec2c88bda2180a6e470ae71f7aa7a
(cherry picked from commit 160f7592f2364b47c615cff96117f0877d58e427)
Signed-off-by: Paul Lawrence <paullawrence@google.com>