ext4: fix overflow in ext4_iomap_alloc()

ext4: fix overflow in ext4_iomap_alloc()

commit d0b040f5f2557b2f507c01e88ad8cff424fdc6a9 upstream.

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: 378f32bab371 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>