vm_fault: drop never-true busy_sleep test

vm_fault_busy_sleep() tests to see whether the page fs->m matches the
value looked up at fs->pindex. At that point, a lock is held on
fs->object, and it has been held since before vm_fault_object() also
looked up fs->pindex in fs->object and stored the result in fs->m. So
the values must match, and the test is not necessary. Drop it.

Reviewed by: alc, markj
Differential Revision: https://reviews.freebsd.org/D51179