Fix printf format warning in zfs_module.c

Clang 5.0.0 got better warnings about print format strings using %zd,
and this leads to the following -Werror warning on e.g. arm:

sys/boot/efi/boot1/zfs_module.c:186:18: error: format specifies type 'ssize_t' (aka 'int') but the argument has type 'off_t' (aka 'long long') [-Werror,-Wformat]
                    "(%lu)\n", st.st_size, spa->spa_name, filepath, EFI_ERROR_CODE(status));
                               ^~~~~~~~~~

Fix this by casting off_t arguments to intmax_t, and using %jd instead.

Reviewed by: tsoome
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D11678