usb: f_mass_storage: Add WARN_ON() when other then SIGUSR1 signal received

Linux USB mass storage function driver creates kernel thread
("file-storage") which handles different signals. On receiving signal
other than SIGUSR1 this thread shall exit. On few instance it is observed
that this thread is being killed. Mass storage function requests for
DELAYED status on set alt (which defers SET_CONFIG process) and sends
exception to this thread which needs to call usb_composite_setup_continue()
API to start control data/status state to get enumeration complete. As
thread is killed, deferred enumeration process is not getting invoke
causing enumeration failure with host (seen as yellow bang on USB composite
device). Hence this change adds WARN_ON() when this condition is detected
and logs process PID who is sending this exception signal to thread.

CRs-Fixed: 819895
Change-Id: Ifb857520bef095e50da24c12ac81b03aba4bbceb
Signed-off-by: Mayank Rana <mrana@codeaurora.org>