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: I251041ff2df61dcc2a818539783cfc0de2e2933a
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@google.com>