ext4: fix overflow in ext4_iomap_alloc()

A code in iomap alloc may overflow block number when converting it to
byte offset. Luckily this is mostly harmless as we will just use more
expensive method of writing using unwritten extents even though we are
writing beyond i_size.

Cc: stable@kernel.org
Fixes: 378f32ba

 ("ext4: introduce direct I/O write using iomap infrastructure")
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210412102333.2676-4-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>