tmpfs supports only INT_MAX nodes due to limitations of unit number allocator.
tmpfs supports only INT_MAX nodes due to limitations of unit number
allocator.

Replace UINT32_MAX checks with INT_MAX. Keeping more than 2^31 nodes in
memory is not likely to become possible in foreseeable feature and would
require new unit number allocator.

Discussed with: delphij
MFC after:	2 weeks

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