ext4: retry block allocation for failed DIO and DAX writes

ext4: retry block allocation for failed DIO and DAX writes

Currently if block allocation for DIO or DAX write fails due to ENOSPC,
we just returned it to userspace. However these ENOSPC errors can be
transient because the transaction freeing blocks has not yet committed.
This demonstrates as failures of generic/102 test when the filesystem is
mounted with 'dax' mount option.

Fix the problem by properly retrying the allocation in case of ENOSPC
error in get blocks functions used for direct IO.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Tested-by: Ross Zwisler <ross.zwisler@linux.intel.com>