this fixes:
fs/fs.c:210:6: warning: symbol 'files' was not declared. Should it be static?
fs/fs.c:212:6: warning: symbol 'get_file' was not declared. Should it be static?
fs/fs.c:227:6: warning: symbol 'put_file' was not declared. Should it be static?
fs/fs.c:248:5: warning: symbol 'dir_is_empty' was not declared. Should it be static?
fs/fs.c:613:41: warning: Using plain integer as NULL pointer

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>