FreeBSD: do_mount() passes wrong string length to helper

FreeBSD: do_mount() passes wrong string length to helper

It should pass `MNT_LINE_MAX`, but passes `sizeof (mntpt)`. This is
harmless because the strlen is not actually used by the helper, but
FreeBSD's Coverity scans complained about it.

This was missed in my audit of various string functions since it is not
actually passed to a string function.

Upon review, it was noticed that the helper function does not need to be
a separate function, so I have inlined it as cleanup.

Reported-by: Coverity (CID 1432079)
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: szubersk <szuberskidamian@gmail.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14136