USB: gadget: f_mass_storage: added eject callback

Added pre_eject() and post_eject() callbacks which are
called before and after removable logical unit is ejected.
The first can prevent logical unit from being ejected.

This commit also changes the way callbacks are passed to
the function from gadget.  A fsg_operations structure has
been created which lists all callbacks -- this is passed
to the fsg_config.

This is important because it changes the way thread_exits()
callback is passed.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>