ocfs2: double evaluation concerns in mlog_errno()

ocfs2: double evaluation concerns in mlog_errno()

It won't happen in real life, but for consistency etc then we should
only evaluate the "st" parameter once.

Also, since not all callers use the new return, it causes at static
checker warning:
fs/ocfs2/export.c:138 ocfs2_get_dentry() warn: unchecked 'PTR_ERR'

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: alex chen <alex.chen@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>