Do not read the on-disk inode in case of vnode allocation.

Reported by: Christopher Krah <krah@protonmail.com>
Reported as: FS-6-EXT2-4: Denial Of Service in mkdir-0 (ext2_mkdir/vn_rdwr)
Reviewed by: pfg
MFC after: 1 week

Differential Revision: https://reviews.freebsd.org/D19327