By doing file extension fast, it is possible to create excess supply

of the D_NEWBLK kinds of dependencies (i.e. D_ALLOCDIRECT and
D_ALLOCINDIR), which can exhaust kmem.

Handle excess of D_NEWBLK in the same way as excess of D_INODEDEP and
D_DIRREM, by scheduling ast to flush dependencies, after the thread,
which created new dep, left the VFS/FFS innards.  For D_NEWBLK, the
only way to get rid of them is to do full sync, since items are
attached to data blocks of arbitrary vnodes.  The check for D_NEWBLK
excess in softdep_ast_cleanup_proc() is unlocked.

For 32bit arches, reduce the total amount of allowed dependencies by
two.  It could be considered increasing the limit for 64 bit platforms
with direct maps.

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks

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