efi: Make efivarfs entries immutable by default

efi: Make efivarfs entries immutable by default

commit ed8b0de5a33d2a2557dce7f9429dca8cb5bc5879 upstream.

"rm -rf" is bricking some peoples' laptops because of variables being
used to store non-reinitializable firmware driver data that's required
to POST the hardware.

These are 100% bugs, and they need to be fixed, but in the mean time it
shouldn't be easy to *accidentally* brick machines.

We have to have delete working, and picking which variables do and don't
work for deletion is quite intractable, so instead make everything
immutable by default (except for a whitelist), and make tools that
aren't quite so broad-spectrum unset the immutable flag.

Signed-off-by: Peter Jones <pjones@redhat.com>
Tested-by: Lee, Chun-Yi <jlee@suse.com>
Acked-by: Matthew Garrett <mjg59@coreos.com>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
[ luis: backported to 3.16:
  - use mutex_lock/unlock() instead of inode_lock/unlock()
  - use root->d_inode instead of d_inode()  ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>