xfs: remove unnecessary shutdown check from xfs_iflush()

The shutdown check in xfs_iflush() duplicates checks down in the
buffer code. If the fs is shut down, xfs_trans_read_buf_map() always
returns an error and falls into the same error path. Remove the
unnecessary check along with the warning in xfs_imap_to_bp()
that generates excessive noise in the log if the fs is shut down.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>