reiserfs: get rid of AOP_FLAG_CONT_EXPAND flag

Remove usage of AOP_FLAG_CONT_EXPAND flag. Reiserfs is the only user of
it and it is easy to avoid.

Link: https://lore.kernel.org/r/20220220232219.1235-1-edward.shishkin@gmail.com
Signed-off-by: Edward Shishkin <edward.shishkin@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>