kernel - Fix cpu/token starvation, vfs_busy deadlocks. incls sysctl

* Remove the mplock around the userland sysctl system call, it should no
  longer be needed.

* Remove the mplock around getcwd(), it should no longer be needed.

* Change the vfs_busy(), sys_mount(), and related mount code to use the
  per-mount token instead of the mp lock.

* Fix a race in vfs_busy() which could cause it to never get woken up.

* Fix a deadlock in nlookup() when the lookup is racing an unmount.  When
  the mp is flagged MNTK_UNMOUNT, the unmount is in progress and the lookup
  must fail instead of loop.

* per-mount token now protects mp->mnt_kern_flag.

* unmount code now waits for final mnt_refs to return to the proper value,
  fixing races with other code that might temporarily ref the mount point.

* Add lwkt_yield()'s in nvtruncbuf*() and nvnode_pager_setsize(), reducing
  cpu stalls due to large file-extending I/O's.  Also in tmpfs.

* Use a marker in the vm_meter code and check for vmobj_token collisions.
  When a collision is detected, give other threads a chance to take the
  token.  This prevents hogging of this very important token.

Testing-by: dillon, vsrinivas, ftigeot
bce: RX and TX ring counts are not required to be same

However, in Dragonfly, RX ring count must be great than TX ring count.

Clue-from: Linux bnx2
icmp: ICMP is MPSAFE
kernel - Remove remaining mplock use cases from tmpfs

* Use the per-mount lock for remaining cases, including nremove,
  truncate, and other operations.

* Also fixes machine stalls against pings when removing very
  large files.

Submitted-by: vsrinivas
kernel - cleanup

* Minor cleanup
kernel - add yields in the swap pager freeing path

* Add yields in swp_pager_meta_free*().  This routine can loop
  heavily on very large VM objects and we don't want it to stall
  the cpu.