ext4: fix the number of credits needed for ext4_unlink() and ext4_rmdir()

The ext4_unlink() and ext4_rmdir() don't actually release the blocks
associated with the file/directory.  This gets done in a separate jbd2
handle called via ext4_evict_inode().  Thus, we don't need to reserve
lots of journal credits for the truncate.

Note that using too many journal credits is non-optimal because it can
leading to the journal transmit getting closed too early, before it is
strictly necessary.

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