ext4: use search_dirblock() in ext4_dx_find_entry()

Use the search_dirblock() in ext4_dx_find_entry().  It makes the code
easier to read, and it takes advantage of common code.  It also saves
100 bytes or so of text space.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Brad Spengler <spender@grsecurity.net>