In selfdfree re-evaulate sf_si after takin the lock.

Otherwise we can race with doselwakeup.

This is a fixup to r273549

Reviewed by:	jhb
Reported by:	everyone and their dog


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273555 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f