nfs: fix incorrect handling of large-number NFS errors in nfs4_do_mkdir()

A recent commit introduced nfs4_do_mkdir() which reports an error from
nfs4_call_sync() by returning it with ERR_PTR().

This is a problem as nfs4_call_sync() can return negative NFS-specific
errors with values larger than MAX_ERRNO (4095).  One example is
NFS4ERR_DELAY which has value 10008.

This "pointer" gets to PTR_ERR_OR_ZERO() in nfs4_proc_mkdir() which
chooses ZERO because it isn't in the range of value errors.  Ultimately
the pointer is dereferenced.

This patch changes nfs4_do_mkdir() to report the dentry pointer and
status separately - pointer as a return value, status in an "int *"
parameter.

The same separation is used for _nfs4_proc_mkdir() and the two are
combined only in nfs4_proc_mkdir() after the status has passed through
nfs4_handle_exception(), which ensures the error code does not exceed
MAX_ERRNO.

It also fixes a problem in the even when nfs4_handle_exception() updated
the error value, the original 'alias' was still returned.

Reported-by: Anna Schumaker <anna@kernel.org>
Fixes: 8376583b ("nfs: change mkdir inode_operation to return alternate dentry if needed.")
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>