ext4: fix ext4 so it works without CONFIG_PROC_FS

This fixes a bug which was introduced in dd68314c.  The problem
came from the test of the return value of proc_mkdir which is always
false without procfs, and this would initialization of ext4.

Signed-off-by: Fabrice Jouhaud <yargil@free.fr>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>