ext4: don't pass freed handle to ext4_walk_page_buffers

This is harmless, since ext4_walk_page_buffers only passes the handle
onto the callback function, and in this call site the function in
question, bput_one(), doesn't actually use the handle.  But there's no
point passing in an invalid handle, and it creates a Coverity warning,
so let's just clean it up.

Addresses-Coverity-Id: #1091168

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