efivarfs: Revert "allow creation of zero length files"

As agreed with the fwupd/LVFS maintainer, this reverts commit
fc20737d again for the v6.15 cycle,
leaving them sufficient time to roll out a fix for the issue that the
reverted commit works around.

Link: https://lore.kernel.org/all/63837c36eceaf8cf2af7933dccca54ff4dd9f30d.camel@HansenPartnership.com/


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>