In kern_linkat() and kern_renameat(), do not call namei(9) while
holding a write reference on the filesystem.  Try to get write
reference in unblocked way after all vnodes are resolved; if failed,
drop all locks and retry after waiting for suspension end.

The VFS_UNMOUNT() methods for UFS and tmpfs try to establish
suspension on unmount, while covered vnode is locked by VFS, which
prevents namei() from stepping over the mount point.  The thread doing
namei() sleeps on the covered vnode lock, owning the write ref.

Reported by:	bdrewery
Tested by:	bdrewery (previous version), pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Fix fcntl(2) compat32 after r270691.  The copyin and copyout of the
struct flock are done in the sys_fcntl(), which mean that compat32 used
direct access to userland pointers.

Move code from sys_fcntl() to new wrapper, kern_fcntl_freebsd(), which
performs neccessary userland memory accesses, and use it from both
native and compat32 fcntl syscalls.

Reported by:	jhibbits
Sponsored by:	The FreeBSD Foundation
MFC after:	3 days