ext4: Fix Kconfig typo for ext4dev

Looks like there is one more instance where ext4dev should be changed
to ext4 because the module name will be "ext4" unless EXT4DEV_COMPAT
is selected.

Signed-off-by: Manish Katiyar <mkatiyar@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>