exportfs: fix 'passing zero to ERR_PTR()' warning

Fix a static code checker warning:
  fs/exportfs/expfs.c:171 reconnect_one() warn: passing zero to 'ERR_PTR'

The error path for lookup_one_len_unlocked failure
should set err to PTR_ERR.

Fixes: bbf7a8a3 ("exportfs: move most of reconnect_path to helper function")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>