ext4: don't set up encryption key during jbd2 transaction

In the Linux kernel, the following vulnerability has been resolved:

ext4: don't set up encryption key during jbd2 transaction

Commit a80f7fcf1867 ("ext4: fixup ext4_fc_track_* functions' signature")

extended the scope of the transaction in ext4_unlink() too far, making

it include the call to ext4_find_entry(). However, ext4_find_entry()

can deadlock when called from within a transaction because it may need

to set up the directory's encryption key.

Fix this by restoring the transaction to its original scope.