efi_loader: buffer size for load options

The number of bytes in an utf-8 string is an upper limit for the number of
words in the equivalent utf-16 string. In so far the inumbant coding works
correctly. For non-ASCII characters the utf-16 string is shorter. With the
patch only the necessary buffer size is allocated for the load options.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>