MFC
rev. 1.232, 1.233, 1.240 of the src/sys/kern/vfs_mount.c, and
rev. 1.215 of src/sys/sys/mount.h (with change to keep the RELENG_6 ABI).

Fix the races that could occur while waiting for the coveredvp lock.

Approved by: re (kensmith), pjd (mentor)