efivarfs: prevent setting of zero size on the inodes in the cache

Current efivarfs uses simple_setattr which allows the setting of any
size in the inode cache.  This is wrong because a zero size file is
used to indicate an "uncommitted" variable, so by simple means of
truncating the file (as root) any variable may be turned to look like
it's uncommitted.  Fix by adding an efivarfs_setattr routine which
does not allow updating of the cached inode size (which now only comes
from the underlying variable).

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