Konstantin Belousov 548066ea66 The quotactl, statfs and fstatfs syscall implementations may dereference
NULL pointer to struct mount if the looked up vnode is reclaimed. Also,
these syscalls only mnt_ref() the mp, still allowing it to be unmounted;
only struct mount memory is kept from being reused.

Lock the vnode when doing name lookup, then reference its mount point,
unlock the vnode and vfs_busy the mountpoint. This sequence shall take
care of both races.

Reported and tested by:	pho
Discussed with:	attilio
MFC after:	1 month
2008-12-18 12:01:19 +00:00
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