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

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

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D52486