ext4: fix potential buffer head leak when add_dirent_to_buf() returns ENOSPC

Previously add_dirent_to_buf() did not free its passed-in buffer head
in the case of ENOSPC, since in some cases the caller still needed it.
However, this led to potential buffer head leaks since not all callers
dealt with this correctly.  Fix this by making simplifying the freeing
convention; now add_dirent_to_buf() *never* frees the passed-in buffer
head, and leaves that to the responsibility of its caller.  This makes
things cleaner and easier to prove that the code is neither leaking
buffer heads or calling brelse() one time too many.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Curt Wohlgemuth <curtw@google.com>
Cc: stable@kernel.org