efivarfs: fix statfs() on efivarfs

Some firmware (notably U-Boot) provides GetVariable() and
GetNextVariableName() but not QueryVariableInfo().

With commit d86ff333 ("efivarfs: expose used and total size") the
statfs syscall was broken for such firmware.

If QueryVariableInfo() does not exist or returns EFI_UNSUPPORTED, just
report the file system size as 0 as statfs_simple() previously did.

Fixes: d86ff333 ("efivarfs: expose used and total size")
Link: https://lore.kernel.org/all/20230910045445.41632-1-heinrich.schuchardt@canonical.com/


Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
[ardb: log warning on QueryVariableInfo() failure]
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>