ext4 crypto: simplify interfaces to directory entry insert functions

A number of functions include ext4_add_dx_entry, make_indexed_dir,
etc. are being passed a dentry even though the only thing they use is
the containing parent.  We can shrink the code size slightly by making
this replacement.  This will also be useful in cases where we don't
have a dentry as the argument to the directory entry insert functions.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>