ext4: Add select for CONFIG_FS_IOMAP

When ext4 is compiled with DAX support, it now needs the iomap code. Add
appropriate select to Kconfig.
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/Kconfig

@@ -37,6 +37,7 @@ config EXT4_FS
 	select CRC16
 	select CRYPTO
 	select CRYPTO_CRC32C
+	select FS_IOMAP if FS_DAX
 	help
 	  This is the next generation of the ext3 filesystem.