libzfs: run_process: reuse line, don't leak it

line will grow as wide as it needs (glibc starts off at 120),
we can store a narrower view; this also fixes leaks in a few scenarios

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12082