USB: g_file_storage: "fsg_" prefix added to some identifiers

Prefixed some identifiers that were defined in storage_common.c file
with "fsg_".  Not all identifiers were prefixed but the ones that are
most likely to produce conflicts when used with other USB functions.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>