ext4: start handle at the last possible moment in ext4_unlink()

ext4: start handle at the last possible moment in ext4_unlink()

Don't start the jbd2 transaction handle until after the directory
entry has been found, to minimize the amount of time that a handle is
held active.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>