xfs: use shmem_kernel_file_setup in xfile_create

shmem_kernel_file_setup is equivalent to shmem_file_setup except that it
already sets the S_PRIVATE flag.  Use it instead of open coding the
logic.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>