fs: ext4: Fix journal overrun issue reported by Coverity

fs: ext4: Fix journal overrun issue reported by Coverity

While &p_jdb[fs->blksz] is a valid expression (it points *one* char
sized element past the end of the array, e.g. &p_jdb[fs->blksz + 1] is
invalid (according to the C standard (C99/C11)).

Changing this to tag = (struct ext3_journal_block_tag *)(p_jdb + ofs);

Cc: Stefan Brüns <[email protected]>
Suggested-by: Stefan Brüns <[email protected]>
Reported-by: Coverity (CID: 165117, 165110)
Signed-off-by: Tom Rini <[email protected]>
Reviewed-by: Stefan Brüns <[email protected]>