Fix a DIV0 in case a large value for fs_avgfilesize or fs_avgfpdir is given (with newfs or tunefs) and dirsize overflows.
Fix a DIV0 in case a large value for fs_avgfilesize or fs_avgfpdir
is given (with newfs or tunefs) and dirsize overflows.

In case dirsize is <= 0 because of an overflow set maxcontigdirs
to 0 so it will be 1 later. This is what would happen for large
fs_avgfilesize. [1]

Identified with help from:	roberto, pjd
Submitted by:			pjd [1]
Approved by:			re (rwatson)
MFC after:			8 days

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