ext4: return an error if kset_create_and_add fails in ext4_init_fs()

In the very unlikely case that kset_create_and_add() fails when the
ext4.ko module is being loaded (or during kernel startup) set err so
that it's clear that the module load failed.

https://bugzilla.kernel.org/show_bug.cgi?id=27912

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>