vfs: manage mnt_writeopcount with atomics

[mjg]

Summary

This requires a small hack in SU code which disables suspension "by hand". I did not think it's worthwhile making it use a generic routine.

Tested by pho

[mjg]

• rebase on top of the custom barrier

[kib]

sys/kern/vfs_vnops.c

1638

I think you can write
if (__predict_true(!mplocked && (flags & V_XSLEEP) == 0) && vfs_op_thread_enter(mp)) {
and remove that ugly slowpath label.

[mjg]

• remove the slowpath label

[mjg]

• fix parens placement

[jeff]

sys/kern/vfs_vnops.c

1800–1802

Since we do this twice it could just be part of a macro or inline.