LU-11535 ldiskfs: allocate extra ldiskfs_ext_path for root

LU-11535 ldiskfs: allocate extra ldiskfs_ext_path for root

Patch ext4_s_max_ext_tree_depth changes path array allocation.
Maximum extent depth is counted in ext4_ext_init(), but
extent's root stored in i_data is not counted. This leads to
out of array writting in ldiskfs_ext_remove_space() and following
fault during transaction commit:

BUG: unable to handle kernel NULL pointer dereference at (null)
[<ffffffffa0f25acb>] osd_trans_commit_cb+0xcb/0x2b0 [osd_ldiskfs]
[<ffffffffa0ecc8e1>] ldiskfs_journal_commit_callback+0x61/0x80
[<ffffffffa03eb8ef>] jbd2_journal_commit_transaction+0x116f/0x15a0

This patch adds one extra element for root in path array.

Cray-bug-id: LUS-6488
Signed-off-by: Artem Blagodarenko <artem.blagodarenko@gmail.com>
Change-Id: I950e223f6ad68c88c1e78fc62448542fd4e78329
Reviewed-on: https://review.whamcloud.com/33388
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>