ext4: Change debugging support help prefix from EXT4 to Ext4

All other configuration options for the ext* family of file systems use
"Ext%u" instead of "EXT%u".

Fixes: 6ba495e9

 ("ext4: Add configurable run-time mballoc debugging")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>