ext4 crypto: clean up error handling in ext4_fname_setup_filename

Fix a potential memory leak where fname->crypto_buf.name wouldn't get
freed in some error paths, and also make the error handling easier to
understand/audit.

Change-Id: I06788ff4a742aacb72eda1bc36371fc23489d55c
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: "Theodore Ts'o" <tytso@google.com>