efi: efivars: Fix variable writes with unsupported query_variable_store()

Commit 8a254d90 ("efi: efivars: Fix variable writes without
query_variable_store()") addressed an issue that was introduced during
the EFI variable store refactor, where alternative implementations of
the efivars layer that lacked query_variable_store() would no longer
work.

Unfortunately, there is another case to consider here, which was missed:
if the efivars layer is backed by the EFI runtime services as usual, but
the EFI implementation predates the introduction of QueryVariableInfo(),
we will return EFI_UNSUPPORTED, and this is no longer being dealt with
correctly.

So let's fix this, and while at it, clean up the code a bit, by merging
the check_var_size() routines as well as their callers.

Cc: <stable@vger.kernel.org> # v6.0
Fixes: bbc6d2c6

 ("efi: vars: Switch to new wrapper layer")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Aditya Garg <gargaditya08@live.com>