fsnotify: return non const from fsnotify_data_inode()

Return non const inode pointer from fsnotify_data_inode().
None of the fsnotify hooks pass const inode pointer as data and
callers often need to cast to a non const pointer.

Link: https://lore.kernel.org/r/20200708111156.24659-3-amir73il@gmail.com


Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>