Allow mounting of usbfs.

Addresses denials such as:
avc:  denied  { mount } for  pid=5 comm="kworker/u:0" name="/" dev=usbfs ino=3234 scontext=u:r:kernel:s0 tcontext=u:object_r:usbfs:s0 tclass=filesystem

Change-Id: I1db52193e6a2548c37a7809ef44cf7fd3357326d
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>