MFC: r344755:
Fix integer overflow possibility.

Reported by: Christopher Krah, Thomas Barabosch, and Jan-Niclas Hilgert of Fraunhofer FKIE
Reported as: FS-2-EXT2-1: Out-of-Bounds Write in nmount (ext2_vget)
Reviewed by: pfg

Differential Revision: https://reviews.freebsd.org/D19326