pybind/cephfs: fix missing terminating NULL char in readlink()'s C string

pybind/cephfs: fix missing terminating NULL char in readlink()'s C string

The error is in test_readlink() in src/test/pybind/test_cephfs.py:

...
test_cephfs.test_readlink ... FAIL

======================================================================
FAIL: test_cephfs.test_readlink
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/tmhoang/test_cephfs.py", line 99, in test_readlink
    assert_equal(d, b"/file-1")
AssertionError: b'/file-1\xe8' != b'/file-1'
...

Fixes: https://tracker.ceph.com/issues/48408
Signed-off-by: Tuan Hoang <tmhoang@linux.ibm.com>
(cherry picked from commit b7af5152c1be806fd529242411b9583fd4d50c76)