efivarfs: make variable_is_present use dcache lookup

Instead of searching the variable entry list for a variable, use the
dcache lookup functions to find it instead.  Also add an efivarfs_
prefix to the function now it is no longer static.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>