udf: Fix reading of in-ICB files

After merging address space operations of normal and in-ICB files,
readahead could get called for in-ICB files which resulted in
udf_get_block() being called for these files. udf_get_block() is not
prepared to be called for in-ICB files and ends up returning garbage
results as it interprets file data as extent list. Fix the problem by
skipping readahead for in-ICB files.

Fixes: 37a8a39f

 ("udf: Switch to single address_space_operations")
Signed-off-by: Jan Kara <jack@suse.cz>