ubifs: fix snprintf() length check

The snprintf() function returns the number of bytes (not including the
NUL terminator) which would have been printed if there were enough
space.  So it can be greater than UBIFS_DFS_DIR_LEN.  And actually if
it equals UBIFS_DFS_DIR_LEN then that's okay so this check is too
strict.

Fixes: 9a620291fc01 ("ubifs: Export filesystem error counters")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Richard Weinberger <richard@nod.at>