MFC: r344757:
Fix double free in case of mount error.

Reported by: Christopher Krah, Thomas Barabosch, and Jan-Niclas Hilgert of Fraunhofer FKIE
Reported as: FS-9-EXT3-2: Denial Of Service in nmount-5 (vm_fault_hold)
Reviewed by: pfg

Differential Revision: https://reviews.freebsd.org/D19385