ext4: handle errors in ext4_rename

Checking return code from ext4_journal_get_write_access() is important
with snapshots, because this function invokes COW, so may return new
errors, such as ENOSPC.

We move the call to ext4_journal_get_write_access earlier in the
function, to simplify error handling in the case that this function
returns returns an error.

Signed-off-by: Amir Goldstein <amir73il@users.sf.net>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>