fsnotify: move declaration of fsnotify_mark_connector_cachep to fsnotify.h

Move fsnotify_mark_connector_cachep to fsnotify.h to properly
share it with the user in mark.c and avoid the following warning
from sparse:

fs/notify/mark.c:82:19: warning: symbol 'fsnotify_mark_connector_cachep' was not declared. Should it be static?

Link: https://lore.kernel.org/r/20191015132518.21819-1-ben.dooks@codethink.co.uk


Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Jan Kara <jack@suse.cz>