ext4: fix estimate extent index blocks in ext4_ext_index_trans_blocks()

The estimate of the number of impacted extent tree index blocks could be
one-too-low. If we modify say 2 extents, already two leaf index blocks
could be impacted, not just one the current estimate counts with. Fix
the estimate.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260805153605.166545-6-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>