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lock: avoid unintentional lock acquisition at failure of readlock

Acquiring lock by vfs.makelock() and getting lock holder (aka
"locker") information by vfs.readlock() aren't atomic action.
Therefore, failure of the former doesn't ensure success of the latter.

Before this patch, lock is unintentionally acquired, if
self.parentlock is None (this is default value), and lock._readlock()
returns None for ENOENT at vfs.readlock(), because these None are
recognized as equal to each other.

In this case, lock symlink (or file) isn't created, even though lock
is treated as acquired in memory.

To avoid this issue, this patch retries lock acquisition immediately,
if lock._readlock() returns None "locker".

This issue will be covered by a test added in subsequent patch,
because simple emulation of ENOENT at vfs.readlock() easily causes
another issue.  "raising ENOENT only at the first vfs.readlock()
invocation" is too complicated for unit test, IMHO.

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