kern/vfs_vnops.c: generalize the lock primitive for file foffset

[kib]

Summary

Generalize foffset_lock/unlock() by splitting the locking info
file_v_lock/unlock() (LP64 case) or file_v_lock/unlock_mtxp() for ILP32
(using mutex pool) and then taking the action to read the offset.

sys/file.h: rename struct file f_vnread_flags member into generic f_vflags

[kib]

sys/kern/vfs_vnops.c

834

Note that f_offset is no longer read under the sleepq lock in this case, but it does not matter.

[markj]

sys/kern/vfs_vnops.c

818

Should this be state | lock_bit?

[kib]

sys/kern/vfs_vnops.c

810

There too.

[kib]

Fix CAS.

[markj]

sys/kern/vfs_vnops.c

804

813

and below

854

Why not use flagsp here?

[kib]

Remove unneeded volatile qualifier for var accessed as atomic.
Systematically use flagsp.