UFS: make mkdir() and link() reliable when using SU and reaching nlink limit

i_nlink overflow might be transient, i_effnlink indicates the final
value of the link count after all dependencies would be resolved. So if
i_nlink reached the maximum but i_efflink did not, we should be able to
make the link by syncing.

We must sync the whole filesystem to resolve dependencies,
which requires unlocking vnodes locked for VOPs. Use existing
ERELOOKUP/VOP_UNLOCK_PAIR() mechanism to restart the VOP if sync with
unlock was done.

PR: 165392
Reported by: Vsevolod Volkov <vvv@colocall.net>
Reviewed by: mckusick
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D35514