ext4: Allow rename to create more than EXT4_LINK_MAX subdirectories

Use EXT4_DIR_LINK_MAX so that rename() can move a directory into new
parent directory without running into the EXT4_LINK_MAX limit.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>