MFC r312600,r312601,r312602,r312606,r312646:

vfs: refactor _vn_lock

Stop testing for LK_RETRY and error multiple times. Also postpone the
VI_DOOMED until after LK_RETRY was seen as it reads from the vnode.

No functional changes.

vfs: fix whitespace damage in r312600

While here wrap the previously overly long line so that it fits 80 chars.

vfs: __predict_false the need to handle F_HASLOCK

Also reorder the check with DTYPE_VNODE. Passed files are vnodes vast
majority of the time, so it is typically true.

vfs: fix LK_RETRY logic braino in r312600

More style cleanup. Use ANSI C definition for vn_closefile(). Switch
to VNASSERT in _vn_lock(), simplify messages.