ext4: fix potential htree index checksum corruption

In the case where we need to do an interior node split, and
immediately afterwards, we are unable to allocate a new directory leaf
block due to ENOSPC, the directory index checksum's will not be filled
in correctly (and indeed, will not be correctly journalled).

This looks like a bug that was introduced when we added largedir
support.  The original code doesn't make any sense (and should have
been caught in code review), but it was hidden because most of the
time, the index node checksum will be set by do_split().  But if
do_split bails out due to ENOSPC, then ext4_handle_dirty_dx_node()
won't get called, and so the directory index checksum field will not
get set, leading to:

EXT4-fs error (device sdb): dx_probe:858: inode #6635543: block 4022: comm nfsd: Directory index failed checksum

Google-Bug-Id: 176345532
Fixes: e08ac99f

 ("ext4: add largedir feature")
Cc: Artem Blagodarenko <artem.blagodarenko@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>