nfsclient: upgrade vnode lock in VOP_OPEN()/VOP_CLOSE() if we need to flush buffers

VOP_FSYNC() asserts that the vnode is exclusively locked for NFS.
If we try to execute file with recently modified content, the assert is
triggered.

Reviewed by: rmacklem
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32999