ext4: factor out checks from ext4_file_write_iter()

Factor out checks of 'from' and whether we are overwriting out of
ext4_file_write_iter() so that the function is easier to follow.

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