syncer VOP_FSYNC(): unlock syncer vnode around call to VFS_SYNC()

The lock is unneccessary since the mount point is busied, which prevents
unmount and syncer vnode deallocation. Having the vnode locked causes
innocent LoRs and complicates debugging.

Also stop starting write accounting around it. Any caller of
VOP_FSYNC() must do it already, and sync_vnode() does.

Reported and tested by: pho
Reviewed by: markj, mckusick
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D34072