ext4: simplify mpage_add_bh_to_extent()

The argument b_size of mpage_add_bh_to_extent() was bogus since it was
always == blocksize (which we can easily derive from inode->i_blkbits).
Also second branch of condition:
	if (nrblocks >= EXT4_MAX_TRANS_DATA) {
	} else if ((nrblocks + (b_size >> mpd->inode->i_blkbits)) >
						EXT4_MAX_TRANS_DATA) {
	}
was never taken because (b_size >> mpd->inode->i_blkbits) == 1.

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