efivarfs: fix error propagation in efivar_entry_get()

efivarfs: fix error propagation in efivar_entry_get()

efivar_entry_get() always returns success even if the underlying
__efivar_entry_get() fails, masking errors.

This may result in uninitialized heap memory being copied to userspace
in the efivarfs_file_read() path.

Fix it by returning the error from __efivar_entry_get().

Fixes: 2d82e6227ea1 ("efi: vars: Move efivar caching layer into efivarfs")
Cc: <stable@vger.kernel.org> # v6.1+
Signed-off-by: Kohei Enju <kohei@enjuk.jp>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>