NFSD: Clean up nfsd3_proc_create()

As near as I can tell, mode bit masking and setting S_IFREG is
already done by do_nfsd_create() and vfs_create(). The NFSv4 path
(do_open_lookup), for example, does not bother with this special
processing.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>