When restoring the mount after umount failed, the MNTK_UNMOUNT flag

When restoring the mount after umount failed, the MNTK_UNMOUNT flag
prevents insmntque() from placing reallocated syncer vnode on mount
list, that causes panic in vfs_allocate_syncvnode().

Introduce MNTK_NOINSMNTQ flag, that marks the period when instmntque is
not allowed to success, instead of MNTK_UNMOUNT. The MNTK_NOINSMNTQ is
set and cleared simultaneously with MNTK_UNMOUNT, except on umount error
path, where it is cleaned just before the syncer vnode is going to be
allocated.

Reported by:	Peter Jeremy <peterjeremy optushome com au>
Suggested by:	tegge
Approved by:	re (rwatson)