NFSv4.2: Serialise O_DIRECT i/o and fallocate()

Ensure that all O_DIRECT reads and writes complete before calling
fallocate so that we don't race w.r.t. attribute updates.

Fixes: 99f23783 ("NFSv4.2: Always flush out writes in nfs42_proc_fallocate()")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>