[PATCH] ext4: fix credit calculation in ext4_ext_calc_credits_for_insert

Fix a nit in ext4_ext_calc_credits_for_insert().  Besides, credits for the
new root are already added in the index split accounting.

Signed-off-by: Johann Lombardi <johann.lombardi@bull.net>
Signed-off-by: Alex Tomas <alex@clusterfs.com>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>