MFC r349800,r349801:

Fix misc fs fuzzing issues.

Reported by: Christopher Krah, Thomas Barabosch, and Jan-Niclas Hilgert of Fraunhofer FKIE
Reported as: FS-22-EXT2-9: Denial of service in ftruncate-0 (ext2_balloc)

FS-11-EXT2-6: Denial Of Service in write-1 (ext2_balloc)