NFSD: Refactor nfsd_create_setattr()

I'd like to move do_nfsd_create() out of vfs.c. Therefore
nfsd_create_setattr() needs to be made publicly visible.

Note that both call sites in vfs.c commit both the new object and
its parent directory, so just combine those common metadata commits
into nfsd_create_setattr().

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