ext2: Annotate implicit fall through in __ext2_truncate_blocks

There is a plan to build the kernel with -Wimplicit-fallthrough and
these places in the code produced warnings (W=1).

This commit removes the following warnings:

  fs/ext2/inode.c:1237:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
  fs/ext2/inode.c:1244:7: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Jan Kara <jack@suse.cz>