zdb: Handle theoretical buffer overflow when printing float

CodeQL pointed out that for extreme floating point values, sprintf()
will overwrite a 32 character buffer. It cited 1e304 as an example,
which causes sprintf() to print 308 characters.

In practice, the numbers should never exceed 100, so this should not
happen. To silence the warning and also handle unexpected situations, we
change the code to use snprintf().

This was missed during my audit of our use of sprintf(), since I did
not think to consider extreme floating point representations. It also
really should not happen, so this change is purely defensive
programming.

This was found by CodeQL's cpp/overrunning-write-with-float check.

Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14264