MFC 287833:

MFC 287833:
Threads holding a read lock of a sleepable rm lock are not permitted
to sleep.  The rmlock implementation enforces this by disabling
sleeping when a read lock is acquired. To simplify the implementation,
sleeping is disabled for most of the duration of rm_rlock.  However,
it doesn't need to be disabled until the lock is acquired.  If a
sleepable rm lock is contested, then rm_rlock may need to acquire the
backing sx lock.  This tripped the overly-broad assertion.  Fix by
relaxing the assertion around the call to sx_xlock().

git-svn-id: svn://svn.freebsd.org/base/stable/10@288958 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f