ext4: generalize EXT4_GET_BLOCKS_IO_SUBMIT flag usage

ext4: generalize EXT4_GET_BLOCKS_IO_SUBMIT flag usage

Currently, the EXT4_GET_BLOCKS_IO_SUBMIT flag is only used during data
writeback to indicate that in ordered mode, the journal commit thread
should skip re-submitting data and simply wait for I/O completion.

To prepare for later patches that need to detect I/O submission context
in ext4_map_blocks(), generalizes the meaning of
EXT4_GET_BLOCKS_IO_SUBMIT. This flag will be set during:
 1) data I/O writeback,
 2) I/O completion extents conversion,
 3) journal performing commit in fast_commit.

This change doesn't affect current usage of this flag and provides a
clear way to identify I/O submission context.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20250423085257.122685-3-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>