vfs: add accessors to ->v_irflag

[mjg]

Summary

The v_irflag field is mostly accessed in an unlocked manner, but no measures were taken to prevent compiler mischief. While here this adds assertions as to state of the field.

[mjg]

sys/kern/vfs_subr.c

1824

This is an artifact from coccinelle. Will patch later.

[kib]

sys/fs/nullfs/null_subr.c

272–273

May be add vn_irflag_set_unlocked() that does VI_LOCK/set/VI_UNLOCK.

sys/sys/vnode.h

990

!= 0

sys/ufs/ufs/ufs_vnops.c

288

This should be _set_cond.

[mjg]

• better assert
• more accessors
• rebase