nfs: deny any concurrent access to a file opened for write

[kofemann]

Motivation:
dCache design flow allows concurrent writes for a file as long as it in create-only mode. Such a flow typically ends with a two replicas of a file with a same pnfsid, but different contents:

$ python
>>> f1 = open("file.txt", "w")
>>> f2 = open("file.txt", "w")
>>> f1.write("aaaa")
4
>>> f2.write("bbb")
3
>>> f1.close()
>>> f2.close()

Modification:
Enforce share-deny for files opened for write.

Result:

$ python
>>> f1 = open("file.txt", "w")
>>> f2 = open("file.txt", "w")
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    f2 = open("file.txt", "w")
PermissionError: [Errno 13] Permission denied: 'file.txt'

>>> f2 = open("file.txt", "r")
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    f2 = open("file.txt", "r")
PermissionError: [Errno 13] Permission denied: 'file.txt'

Acked-by: Marina Sahakyan
Target: master, 12.0, 11.2
Require-book: no
Require-notes: yes
(cherry picked from commit 5bd6b33)