Cleanup dump_bookmarks()

Assertions are meant to check assumptions, but the way that this
assertion is written does not check an assumption, since it is provably
always true. Removing the assertion will cause a compiler warning (made
into an error by -Werror) about printing up to 512 bytes to a 256-byte
buffer, so instead, we change the assertion to verify the assumption
that we never do a snprintf() that is truncated to avoid overrunning the
256-byte buffer.

This was caught by an audit of the codebase to look for misuse of
snprintf() after CodeQL reported that we had misused snprintf(). An
explanation of how snprintf() can be misused is here:

https://www.redhat.com/en/blog/trouble-snprintf

This particular instance did not misuse snprintf(), but it was caught
by the audit anyway.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14098