linprocfs: Properly reset error variable for mtab generation

Both functions linprocfs_domtab() and linprocfs_doprocmountinfo() are
logically identical, but the former fails with ECANCELED because error
is not reset after the for loop.

Reviewed by: jrm, fluffy
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47865

(cherry picked from commit 58c7db14cd71c41f59d80d26d921782c0c27d523)