lockf: ensure atomicity of lockf for open(O_CREAT|O_EXCL|O_EXLOCK)

or EX_SHLOCK. Do it by setting a vnode iflag indicating that the locking
exclusive open is in progress, and not allowing F_LOCK request to make
a progress until the first open finishes.

Requested by: mckusick
Reviewed by: markj, mckusick
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28697