ext4: fix offset overflow on 32-bit archs in ext4_iomap_begin()

ext4_iomap_begin() has a bug where offset returned in the iomap
structure will be truncated to unsigned long size. On 64-bit
architectures this is fine but on 32-bit architectures obviously not.
Not many places actually use the offset stored in the iomap structure
but one of visible failures is in SEEK_HOLE / SEEK_DATA implementation.
If we create a file like:

dd if=/dev/urandom of=file bs=1k seek=8m count=1

then

lseek64("file", 0x100000000ULL, SEEK_DATA)

wrongly returns 0x100000000 on unfixed kernel while it should return
0x200000000. Avoid the overflow by proper type cast.

Fixes: 545052e9e35a ("ext4: Switch to iomap for SEEK_HOLE / SEEK_DATA")
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org # v4.15

fs/ext4/inode.c

@@ -3524,7 +3524,7 @@ retry:
 		iomap->flags |= IOMAP_F_DIRTY;
 	iomap->bdev = inode->i_sb->s_bdev;
 	iomap->dax_dev = sbi->s_daxdev;
-	iomap->offset = first_block << blkbits;
+	iomap->offset = (u64)first_block << blkbits;
 	iomap->length = (u64)map.m_len << blkbits;
 
 	if (ret == 0) {