Handle excess of D_NEWBLK in the same way as excess of D_INODEDEP and
D_DIRREM, by scheduling ast to flush dependencies.
For 32bit arches, reduce the total amount of allowed dependencies by two.
MFC r287479:
Declare the writes around the call to VFS_SYNC() in
softdep_ast_cleanup_proc().
MFC r287483:
Do not consume extra reference.