ext4: Allow parallel DIO reads

We can easily support parallel direct IO reads. We only have to make
sure we cannot expose uninitialized data by reading allocated block to
which data was not written yet, or which was already truncated. That is
easily achieved by holding inode_lock in shared mode - that excludes all
writes, truncates, hole punches. We also have to guard against page
writeback allocating blocks for delay-allocated pages - that race is
handled by the fact that we writeback all the pages in the affected
range and the lock protects us from new pages being created there.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>