ext4: mechanical rename some of the direct I/O get_block's identifiers

This commit renames some of the direct I/O's block allocation flags,
variables, and functions introduced in Mingming's "Direct IO for holes
and fallocate" patches so that they can be used by ext4's buffered
write path as well.  Also changed the related function comments
accordingly to cover both direct write and buffered write cases.

Signed-off-by: Jiaying Zhang <jiayingz@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>