efivarfs: guid part of filenames are case-insensitive

It makes no sense to treat the following filenames as unique,

	VarName-abcdefab-abcd-abcd-abcd-abcdefabcdef
	VarName-ABCDEFAB-ABCD-ABCD-ABCD-ABCDEFABCDEF
	VarName-ABcDEfAB-ABcD-ABcD-ABcD-ABcDEfABcDEf
	VarName-aBcDEfAB-aBcD-aBcD-aBcD-aBcDEfaBcDEf
	... etc ...

since the guid will be converted into a binary representation, which
has no case.

Roll our own dentry operations so that we can treat the variable name
part of filenames ("VarName" in the above example) as case-sensitive,
but the guid portion as case-insensitive. That way, efivarfs will
refuse to create the above files if any one already exists.

Reported-by: Lingzhu Xiang <lxiang@redhat.com>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Jeremy Kerr <jeremy.kerr@canonical.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>