dlm: fix dlm_dir_lookup() handling of too long names

... those can happen and BUG() from DLM_ASSERT() in allocate_direntry() is
not a good way to handle them.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Teigland <teigland@redhat.com>