initramfs: fix zpool get argument order

When using the zfs initramfs scripts on my system, I get various
errors at initramfs stage, such as:

cannot open '-o': name must begin with a letter

My zfs binaries are compiled with musl libc, which may be why
this happens. In any case, fix the argument order to make the
zpool binary happy, and to match its --help output.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Daniel Kolesa <daniel@octaforge.org>
Closes #14572