Convert some sprintf() calls to kmem_scnprintf()

These sprintf() calls are used repeatedly to write to a buffer. There
is no protection against overflow other than reviewers explicitly
checking to see if the buffers are big enough. However, such issues are
easily missed during review and when they are missed, we would rather
stop printing rather than have a buffer overflow, so we convert these
functions to use kmem_scnprintf(). The Linux kernel provides an entire
page for module parameters, so we are safe to write up to PAGE_SIZE.

Removing sprintf() from these functions removes the last instances of
sprintf() usage in our platform-independent kernel code. This improves
XNU kernel compatibility because the XNU kernel does not support
(removed support for?) sprintf().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14209