FreeBSD: Fix out of bounds read in zfs_ioctl_ozfs_to_legacy()

There is an off by 1 error in the check. Fortunately, this function does
not appear to be used in kernel space, despite being compiled as part of
the kernel module. However, it is used in userspace. Callers of
lzc_ioctl_fd() likely will crash if they attempt to use the
unimplemented request number.

This was reported by FreeBSD's coverity scan.

Reported-by: Coverity (CID 1432059)
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14135